I seem to have shilled matrix enough for you to consider using it! So now, a brief explanation :D
If I have not shilled you into Matrix yet, you can read why I think it is so great here!
I'll compare matrix to e-mail. With e-mail, lets say if you had an address @gmail.com, you can still mail someone at another server, let's say @yahoo.com. Matrix is the same. If you have an account at :matrix.org, you can still communicate with someone at :mozilla.org.
Also like e-mail, you can use whatever clients you want at any time. And yep, if this is your first time hearing this, you can use any email client you want :)
First, you will need a client. Which one you pick doesn't matter. You can log into them all and keep using the one which you like the best.
If on a computer, I recommend either Cinny or Element. They also have desktop versions if you don't like using them in your browser.
If on a phone, I recommend mainly ElementiOS Google Play. There are a lot less mobile clients :/
There is a comprehensive list of Matrix clients here. (I love Nheko the best ^.^)
So now you have any Matrix client. Great! Pick a name, and a password, and that is it. That simple :D
You can change the homeserver from matrix.org to any other. mozilla.org and kde.org are other popular alternatives. If you want, there's a (mostly) comprehensive list of public homeservers here, but you don't need to get that fancy about it.
How you use Matrix depends on the client. However, you should see buttons to search through a 'room directory', and a button to start a DM. If you want to message me, my MXID(MatriX ID) is @rrogalski:matrix.org. Feel free to look through the public rooms too! Yes, I know the big ones are tech related. But there are ones out there for photography, the harmonica, bicycles, religion, nature, and everything in-between :). Note that the rooms on the top of the room finder are either there because A) most of the users in that room use another service(ie, discord, telegram, etc) or B) it's all just bots.
Matrix supports reactions, custom emojis, stickers, colors, and much more. If it's in any other chat service: we have it in Matrix!