They don’t in general, but things that do heavily detailed graphics work (like your compositor or browser) or lots of cryptography work on the CPU can get a bit more out of those newer instructions than many other programs.

Very approximately, things that Gentoo offers prebuilt versions of because compiling them is so resource intensive are often the things that can get the best benefit out of your architecture variant. (Not singling out Gentoo here as an example of “doing it badly” - they do the sensible thing by providing these prebuilt binaries, but in some ways it defeats the purpose of optimised source distributions.)

It’s a Hard Problem™ to solve.

The irony is that big things like Firefox can get the most advantages from building for your specific CPU variant, especially if you use them frequently.

Yummy!

Discourse, not Discord. The accounts are managed through the same SSO that manages Launchpad accounts, so the devs who will use this already have an account.