I am wondering if there is a discord (ironic i know if there is something equvialent let me know) where I can get into contact with people developing in this space.

I assume there are probably some discords somewhere, but I’d encourage you to communicate on the fediverse about the work you’re doing to improve it. Experience with the fediverse helps people understand the ways in which it can/should improve. And also highlights the ways it NEEDS to improve, if you yourself prefer to use discord even to talk about goings on in the Fediverse, then figuring out WHY you prefer that is probably the first improvement you want to work on.

How does a different architecture like kbin@social ui (and possible backend) communicate with posts from lemmy? Is it through ActivityPub?

Yes, ActivityPub is what allows this, though different apps use ActivityPub in different ways, which pragmatically leads to different levels of useful interoperability.

If you want to learn more about this, I’d recommend setting up a private Lemmy, a private kbin, a private mastodon, and maybe others you’re interested in. Try to federate them and see how it works and what it looks like from each app.

For example, with Mastodon Federating with Lemmy, I’ve heard that a Lemmy community looks like a Mastodon account to follow, posts to the Lemmy community looks like toot threads, and comments look like replies in the toot threads. It feels a little weird as a Lemmy user to see these things out of their normal UI context, but that’s how Mastodon handles the elements of the AcitivtyPub protocol that Lemmy uses. And it works pretty ok for casual interaction with a Lemmy community, though I’d generally prefer using a real Lemmy account to interact with Lemmy communities because it represents these things in a more intuitive way to me.

Could someone from lemmy login to mastodon with their same account? Why or why not?

You can’t use your Mastodon account to log into a Lemmy instance because accounts are bound to an instance. What you CAN do, is use your Mastodon account on your Mastodon instance to “follow” a Lemmy community through federation. Then Lemmy posts will appear in your Mastodon timeline as toot threads, and comments as toots in those threads.

Other fediverse apps may usefully interoperate with Lemmy in different ways, or they may not interoperate in a useful way even though they both use ActivityPub because they use it too differently.

Could two different fediverse apps technically do all the same things if they used the exact protocol to do the same things with a different ui?

Yes, kbin and Lemmy are fairly close to this. I think kbin tries to have a Mastodon-like toot interface as well, though I’m not that familiar.

If someone wanted to go about making their own fediverse app where is a good first learning step?

Good first learning steps include:

• Making accounts on different well-established fediverse apps and using them. I’ve learned the above answers just by doing that.
• Installing your own instances of Fediverse apps you like. This will force you to understand how they work more deeply as you set up federation and see more of their internals.
• Find ways to help existing projects. Right now, there’s a flood of newcomers to Lemmy. Just watching for questions like this and answering takes stress off Lemmy devs and other established community members who are too busy to get to them all. Submit well-formed bug reports, or pull requests that improve performance or fix bugs on existing app projects. Help instance admins debug performance and other issues, or new admins set up their first instance.

I would advise NOT trying to make your own fediverse app. The fediverse is littered with failed vanity projects that either never worked well enough to use, or that worked well and didn’t get used anyway. Much more than new apps, what the fediverse needs is community members populating the existing fediverse apps with interesting things to do and talk about, and developer communities helping the overworked devs of existing projects mature the best fediverse apps faster than the current devs can do alone.