To join from a client you have to go to the "Servers" tab and if you scroll down the featured servers in the bottom you will find a "Add server" button or something like that where you can add a custom server.

Rant: They made sure it's hidden down there because they have partnerships with the sponsored servers which cash in a lot of money from kids going there. They use predatory and aggressive in-game marketing to try and squeeze every cent possible from kid's parents credit cards. I'm glad to see people host their own local servers instead.

https://beebom.com/how-make-minecraft-bedrock-server/

Look at the part about how to join a server.

Usually you can get to a screen that lets you type in a server address. It’s not on the first page. It’s on the Friends tab I think. The Nintendo Switch doesn’t allow you to connect to custom servers at all either.

I know a whole bunch about doing this with the Java version, but absolutely zero about doing it with bedrock.

In the Java version though, it just exposes Minecraft on a specific port and then you just connect your Minecraft client to that port.