How to make a server for Java and Bedrock players

Minecraft comes in two editions that cannot normally play together. A crossplay server fixes that: Java players join the usual way, Bedrock players join through a bridge, and they all share one world.

The pieces

A Java server such as Paper. It holds the world and the rules.
Geyser, a plugin that translates the Bedrock protocol into the Java one, so Bedrock players can join.
Floodgate, a companion plugin that lets Bedrock players sign in with their Xbox account instead of needing a Java account.
ViaVersion (optional), so Java players on newer or older versions can join too.

Steps

Make a Paper server. On Saryno, pick Paper, then switch on Bedrock players under add-ons. Floodgate and ViaVersion come with it. The create page also has a ready-made Java and Bedrock template.
Note two addresses. Java players use the usual address. Bedrock players need the address and a port, which the dashboard shows in the Bedrock card.
Share them. Java: Multiplayer, Add Server. Bedrock: Play, Servers, Add Server, then type the address and the port.
Give it enough memory. Geyser adds weight. Plan on at least 2 GB; 3 GB is comfortable for a small group.

Names and skins

Bedrock players show up with a dot or other prefix in front of their name, so that they never clash with a Java player of the same name. Their skins show up for Java players, and the other way round.

What does not carry over

Crossplay works because Geyser translates, and some things cannot be translated perfectly: a few redstone details, some mod blocks, and exact inventory behaviour. Plugins that add custom items work through Geyser's mapping, and the rest is worth testing before you invite everyone.

When Bedrock players cannot join

Since Bedrock 26.60 the connection method changed, and older Geyser setups stopped working for updated players. See Bedrock 26.60 and NetherNet. On Saryno this is handled for you.

Is it free?

Geyser and Floodgate are free plugins. You need enough memory for them, nothing more. Try it on a free server.