Both are mesh networks, with slight differences. The idea is that volunteers run relay nodes with LORA (which has a range od a few KM, depending on visibility), and you also have client devices, and if you have a large local community of enough nodes and users, you can have an off-grid communication network where data is being sent node to node (both client and relay) before it finds the recipient. Both networks are encrypted.
Most cities already have a pretty good coverage. Meshtastic has a few issues that Meshcore tries to solve, mostly in regards to scaling, but tbh I havent researched it enough to be able to correctly list them (just like this answer is mostly a simplification). There's plenty of blog posts that explain it a lot better.
You can get standalone Meshcore devices (with a screen and keyboard) for around 70$, and devices that connct to your phone through bluetooth and you send messages through the network from an app for even cheaper.
My guess is that it's not entirely adversary-proof, but it probably beats having a phone with you to communicate when you're doing anti-goverment sruff.
And if you're asking about Anarchist Library, there's this site that has a lot of articles, zines and books about good operational security, how to behave on protests, what to (not) bring, first aid against common crowd control, and general anti-goverment guirrella stuff so you can protest as safely as possible.