Deep Dark Dimension

 You can return to the overworld by using the teleporter block. Place it down and right-click it and you'll get transferred back to the overworld. You can either craft the teleporter (which might be impossible, since you can't get netherite and a nether star in the deep dark), or you have to give the teleporter to you via cheats (you find it in the Miscellaneous tab in the creative inventory).

If for some weird reason this does not work, you can type in your command line:

/execute in minecraft:overworld run tp x y z

 This will teleport you to some coordinates x, y and z in the overworld. This is the ultimate last possibility and you should only do this, if the teleporter does not work for you.

It's also important to note, that there's a config file in your config folder named "ddd-common.toml" that contains a bunch of parameters. Somewhere you can find a line

teleporter_block_included = true

 By default, this value should be set to true and it means, that your starter base (if enabled) has a teleporter block to teleport you back. Changing this value to "false" will prevent the mod from adding the teleporter to your starter base. This might be useful, if you don't want the player to be able to teleport back to the overworld anytime. But keep in mind that it might be impossible to craft the teleporter, since some items aren't available in the deep dark!