Ok I guess we have to start way in front of your question to not just answer your question but give you an understanding.

First of all, what does a switch do: A switch has for example 24 ports and all ports are connected to the CPU of the switch. When a pc connected to the switch wants to reach the router to access the internet he asks the switch to really the data to the router but the switch doesn’t know where the router is so it sends on all ports „Hey are you 192.168.0.1(for example if it’s the gateway in your network config)“ then on one port the router answered „yes“ and the switch makes a note on a list that this up is on port x. So over time the switch knows who is where and what speed does the port support. Because said CPU has a bandwidth normally greater than the sum of all ports (rock a Unifi 24 port switch by chance and it had 26gbit bandwidth) it can for example connect 2 pcs transfering files with a full gigabit all while running a download from the internet router to a third pc with no problem. So as long as you don’t want to access one ressource with multiple PCs you have full speed and if you do the packets are stored in a small buffer and if it is full the PCs are told to wait a millisecond and it is transfered in a first come first serve method so everyone can get data through.

All a router does extra on top is have an extra list if he doesn’t find the receiver of the packets internally he notes the address of the PC that asked and sends the request to the wan port and when someone responds he know who to send the response internally.

I usually tell people that data flow is like waterflow in pipes

You have a 100mb internet connection, so your local network splits that between various devices

No. In all cases all devices have maximum speed of your network equipment and themselves. I think what you meant to ask if a switch would reduce the bandwidth available to the devices, which is also a no. Both PC’s using the internet at the same time will reduce the bandwidth, but not 50:50. A normal unconfigured router will give 100% of the bandwidth to each connection, meaning, if PC A is downloading a 1GB file that takes 10’ to download, the internet for all other devices for the next 10’ would appear to be very slow, till that download is finished. A more advanced router, would reduce the bandwidth of PC A to like 70% so that 30% can be used by PC B. In any case, the network equipment you use, as long as it is faster or the same speed as your internet connection, and not connected by dozens of switches, will not slow down your internet. Multiple clients do. I have a network of over 250 clients at home, so there are rules in place to guarantee the bandwidth needed for certain devices (like TV’s for streaming) where as other devices (like phones or tablets) are less prioritized.

The difference between switches and routers used to be very clear. Today what is sold to consumers as routers have built-in switches. And some advanced switches can do routing. But since your topic isn't really related to your question, it's not here or there.

Ethernet - the cables and nics that are commonly used today, is a broadcast network. Meaning every device on the wire can talk without waiting. In the olden days, there was just a wire between all the PCs, no switch - and because there was a limit to the length of that wire, we had "cheap" devices called hubs that simply took X number of plugs and connected them electrically as if it was one long wire. On that wire, only ONE pc could talk at a time. While it talked, everyone else had to listen and wait for silence; then it would try and if nobody else talked it would continue to transmit.

Switches fix that - instead of electrically making it one long wire, it splits each port into its own little network. It's quite normal that only one PC/server is on that line - but it's not really required. It means each PC while still listening to see if there's traffic on the wire before talking, doesn't have to contend with other PCs talking as long as they're not talking to it. A great advantage of this is that you get close to the physical speed limit on each port - the switch having a lot of electronics (a small computer) can forward signals from one port to another. To do this, the switch notes which MAC addresses are present on each port, and it looks at the header of the packages to determine its MAC destination and hence what port to use. The switch, even the 'cheap' ones, can easily forward 100Mbps from one port to another at 100Mbos both ways.

But here's the crock of it - not every device you attach to a switch talks at the same speed. With TCP each package has to be acknowledged, so when a PC isn't ready there's no ACK and hence the sender has to wait. So it may have plenty of bandwidth but it's not going anywhere until the receiver is ready.

If you have a single port on your switch that everything else wants to "share" and transmit through, when that port's bandwidth is at its limit, the senders will be told to wait. It doesn't have to be your cable modem - it can happen if you have a file server on one port, and have everyone in your family trying to view pictures on it at the same time. It's like a water pipe - you can only push a certain amount of water through at a time. It doesn't matter if you split it into several more pipes - if the water has to go through that single pipe, that's your limit.

In advanced networking we can do a lot of things to help determine which traffic should have priority; but the limit is still the limit. It can be made to appear as not making everything slow, by giving priority to the short bursts from user-interactions, while down prioritizing long file transfers and similar.

A gateway is a device that moves traffic between networks - but it's still bound by the same bandwidth issues. Switches are only able to stay within the networks connected to it. A gateway knows which system to send traffic to/via to get to the final destination. They often have few ports - where switches can have a lot - 24, 48 or even more. Gateway devices typically have less than 10 and often less than 5 ports. Unless you have them combined with a switch internally.

Consumer grade devices rarely have advanced features - so it's first come, first serve. As you get more and more smart devices, streaming etc. in your home you'll find you need more advanced networking equipment to avoid issues with everyone wanting to go online at the same time.

You have one pipeline to the Internet that is 100 Mbps wide and it doesn't matter if you put the split of this bandwidth in your router or your switch. At some point all your systems will have to share this 100 Mbps between them.

And this shares are dynamic. If one device currently only uses 10 Mbps, the remaining 90 are free to be used by all the other devices. (highly simplified)

And actually, if your router has more then one lan port, chances are high that this is a switch within your router.

Even if you only have 100 Mbps internet, you still want to make sure your router and switches are 1 Gbps...

it will give you enough overhead in the connections, especially if you still want to transfer data from one device to another on your network.

Scenario 2 is more accurate for BOTH cases, but not because you used a switch. Scenario 1 is false.

It's the ISP provided speed that is the limited resource. You have 100 Mbps of Internet service, which can be consumed by any single connection. If multiple connections want to use Internet, that speed gets split up between all the devices trying to use it simultaneously.

It does not have to be in equal pieces. If one connection only wants 20 Mbps of Internet, another connection can use the other 80 Mbps remaining.

Using a switch does not affect the consumption of Internet from devices unless the switch port speed itself is lower than your maximum ISP speed. This isn't true in your example.