There are many free sites out there, but they don't work very well and have some very restrictive limits. For reliable server hosting, you should either pay for it or host it yourself.

If you're planning on starting a server, you should be prepared to spend money. If it's just for you and your friends, I recommend self-hosting it. There are plenty of tutorials for that, just google them.

There's minehut (which doesn't let you have mods or upload a world) it's not great but if you just want to have a world with less than 10 people you could try using it.

Completely free server hosting is really rare. This is due to the fact the hosting company, either owns or rents "server machines" which costs a lot of money. I'm pretty sure Aternos gives you a free server with limited slots and RAM if you watch adds or something like that. I suggest finding a Minecraft server which features player servers or try hosting from YOUR own computer.