Nobody goes on foot from sea level to the top of the mountain

Tim Macartney-Snape climbed Mount Everest that way. It took him 3 months.
https://en.wikipedia.org/wiki/Tim_Macartney-Snape#Everest:_Sea_to_Summit_Expedition

If you're not starting on foot from the bottom of the Mariana Trench, you're just a filthy casual.

The practical answer is: you drive as far as you legally can.

As a disclaimer, pictured here are the Himalayas, which are at a completely different scale to where I've been, but in my experience there are typically parking spaces/bus stops at the end of public roads. At this point you leave the built up infrastructure and enter nature, and these are often located in a place where the flatter valley ends and a steeper ascent begins. In many cases there are smaller private roads further up to service more remote cabins or farmsteads. Sometimes there are even taxi services that drive you further along using private roads, which can be seen as not fully scaling the mountain yourself. Generally, the closest public parking is considered the starting point and most people will therefore start at the same spot.

For Everest in particular, Base Camp 1 is at about 4000m IIRC, and that's where ascents typically start after some time to acclimate to the already high altitude.

Not a climber but I’m assuming the base of the mountain.

I mean you park the car in the parking lot and then you start at the trailhead. That's really as far as you're allowed to drive up, not like you can drive on the hiking trail

I learned from experience that vehicles lose torque the higher the elevation due to the thinning of oxygen. I think it was something like 30% per kilometer in elevation. That being said I'm sure there's a level where they can't drive anymore.