So there are way more competent, tech-savvy users than me, so I consider myself a layman.

From my understanding is that ActivityPub is basically a foundational, basic level of computer/program coding.

This foundational level of coding tells/ guides software and programs built on top how to communicate with other software built on top, even if the programs have different functions and purposes (e.g. lemmy and mastodon and pixelfield)

So by using a program built using ActivityPub, you can basically use that program to talk, communicate, interact, with a different program built using ActivityPub.

By using lemm.ee (a lemmy instance) you are able to use, post, see and interact with other instances for lemmy as well as other ActivityPub programs like mastodon (foss Twitter) or k/mbin.

So by using lemmy you are using ActivityPub. You can expand your ActivityPub usage by using other ActivityPub programs like pixelfield, mastodon, etc, but theoretically, you should only need one ActivityPub account