There are three ways to play multiplayer in "Minecraft: Java Edition:" through an online server, a LAN server, or a "Minecraft Realms" server.

In Minecraft Java Edition, players can either jump into a world on their own and explore it, or they can join their friends in a multiplayer world and play together.

From this point on, as long as you have the server open on your PC, any player should be able to join after performing these last steps: Open Minecraft: Java Edition. Go to Multiplayer > Add Server.