Thank you for taking the time to help with understanding. You provide a lot of examples which should hit pretty much everyone's current knowledge basis.
If I may try to translate to my knowledge basis. I'm very familiar with #BGP, #TCP/IP and have a deeper than basic use knowledge of #DNS, #SMTP and #HTTPS. Basically, I understand the concept of the Open Systems Interconnection Model (OSI Model) as described here: https://en.wikipedia.org/wiki/OSI_model
In the wiki link, this diagram describes the interoperability idea https://en.wikipedia.org/wiki/File:OSI-model-Communication.svg
So #Mastodon is yet another piece of software that implements parts of the overall #ActivityPub protocol (but not the full extent of the protocol) while attempting to provide the function that non-tech users have come to expect of the bird application?