how do i install a modpack and just 1 plugin?

i just want to install 1 plugin on the modpack

can i access the mod flies in a modpack to update/modify mods?

bruh. Didn't you used to be able to acess the server files in a meaningful way a long time ago?
You can't expect to look up every single mod in the search and then slowly install them 1 by 1...

I hope there will be a way to modify it in the future.