Same Wi-Fi
Open to LAN, from the pause menu of a single-player world, publishes that world on the local network. Friends on the same edition, on the same network, see it in the multiplayer list. It dies when you close your game. It is the right tool for a weekend, and a bad tool for a world you care about, because the host’s computer is the server.
Realms
A Realm is a world Mojang hosts for a monthly fee. You invite players. There is a small set of minigames and a backup slot, which is the feature that matters. Java Realms and Bedrock Realms are separate products. A Realm does not turn a Java world into a Bedrock world. For a family or a club of about ten, it is the least annoying option. Write the rules anyway — the Realms page is that list.
Servers
A server is a program running the world without a player having to sit in the pause menu. Java servers are the famous ones: vanilla from Mojang, or Paper and similar if you want plugins, or Fabric if you want mods. Players join with an address. Bedrock has its own dedicated server, featured servers in the game, and Realms. An address copied from a Java video will not connect from a console.
Big servers add economies, minigames, ranks and anti-cheat. They also add rules beyond Mojang’s. Read them before you place a chest. The short version of what they usually say is on public servers.
Accounts and cross-play
You sign in with a Microsoft account. Java used to have Mojang accounts; the migration finished years ago. If a site still asks for a Mojang password, close it. Official help is help.minecraft.net, and the scam version of this sentence is on staying safe.
Bedrock cross-play is real: a phone can join an Xbox friend, a Windows Bedrock player, a Switch. Java cross-play with those devices is not an official feature. Unofficial proxies exist. They break, they lag, and they are not what Mojang supports. Play together on the edition you all actually own. Which edition that is, and why mods do not follow you, is the editions page.
Where players talk about those servers once they are off them — forums, Discords, SMPs — is fan clubs.