On top of what GunsForBucks has said, some servers require specific mods that all its players must have. Eg: most server-side Valheim+ hosts also requires its players to have it too.

Your best bet is to start your own, or find an existing group.

On top of what Guns and Gins said :

Steam provides the server software free of charge whether you buy the game of not. You can set up your own server , there are many guides available through your favorite search engine or right here on Steam.

You can also just pay a service like gportal $45 for 3 months and you don't have to set-up anything , they give you an ip and port you use to let people join and you can have any rules you want.

I have used both an extra box and the service(s) to host dedicated servers , fairly straight forward.

Game is not an MMO so servers only host 10 people max unmodded. Servers are also hosted by or rented by players not the devs... so there are no public servers in that sense.

People like to know who they play with to avoid griefers etc so it is best to find groups in "looking for groups" or I think they also have a discord thing.

Good luck.