This is Lemmy, on this server (latte.isnot.coffee) we have /c/china and /c/chinavisa and any other communities you want to make which are local to this server and can be interacted with in the same way as Reddit, that's sort of cool but the magic happens elsewhere.
When you signed up for this server, you got [email protected], you can use this to go to different communities on different lemmy servers as a sort of guest and post there too! So you can post anywhere!
How to find other places? I'm going to call them "foreign" for the sake of simplicity - first you can go to Communities -> All (or click here) which will show you the ones the users here locally have subscribed to, and which server they're on (you'll see a lot of lemmy.ml) - if you click that you can go over there and make a post. Think of it as flying to another country
But not all communities are in that list, if no-one here has gone there yet you won't see it.
Do note that you may get 'lost' if you click an offsite link over there, and end up on a different Lemmy server (thus logged out) - you can simply type latte.isnot.coffee again in your URL bar and you'll be right back at home :)
To add an existing 'foreign' community elsewhere to this server, you simply need to search for it from the Communities -> All page. To search you want to use [email protected] (for example to find /c/gaming you'd use [email protected])
You only need to add it once, then everyone on latte.isnot.coffee can see it on the All page. You will need to subscribe to the 'foreign' community to start bringing comments through. Once that's done, you can interact it like you would local communities
Notes: Sometimes it says "No Results" after searching, don't worry about that. You'll simply need to go back to the Communities -> All page and it will be there!
Notes2: You cannot log into your account here from other Lemmy servers, but you can post from here to other lemmy servers. Just make sure your URL is always latte.isnot.coffee
As an example, a post I made to Lemmy Support on lemmy.ml (a foreign server): https://latte.isnot.coffee/post/291
Notes3: You can browse "foreign" communities here: https://browse.feddit.de/ to see what's out there, then...
To join you simply need the community name (let's take asklemmy as an example) and the server. In this case it would be [email protected] to type into the search bar on Communities -> All on latte.isnot.coffee
As the server grows, you may find your favourite communities already in Communities -> All so don't need to worry about searching/subscribing to it.
Notes4: It's not very good at refreshing for new content, you may have to hit F5
I got march of shamed once because I genuinely forgot, it somehow shortcircuits my brain when I need to wait for approval, thinking I've already paid and walk out after waiting for the person to come tap some buttons :|