#properties

Use /gamerule (for example keep_inventory on 1.21.11+, keepInventory on older versions), /difficulty and /gamemode. Set the defaults for new players in Properties.

Set the motd property in the Properties tab (use § colour codes and \n for a second line) and upload a 64×64 server-icon.png to the server's main folder.

Turn off online-mode in the Properties tab, save and restart. Anyone can then join with any name, so add a login plugin to protect accounts.

Stop the server, delete or rename the world folder(s), set a new level-seed in Properties if you want, and start the server to generate a fresh world.

Put a direct download link in resource-pack (plus its SHA-1) in Properties to send a resource pack. Put datapack zips in world/datapacks and reload or restart.

Properties tab → search a setting → flip the switch or type a value → SAVE → restart. Here is what the most useful settings do, and the defaults on a fresh server.