Level Editor - ProBuilder

Unity has a Level Editor that is able to be included free of charge (despite it's name).

Unity will often purchase mods and assets and integrate them into Unity over time.

ProBuilder is the Unity level editor/creator.

Open a project in Unity and the Package Manager.

Make sure that you switch to Unity Registry to see everything that is available.

Install ProBuilder

You can find ProBuilder in the Tools menu.

Open the Shape Editor Menu

This will give you the option to choose different premade shapes to add.

Select a shape to add (in this example a Stair)

When you select a shape you can then see the different options.

Change these to change the shape

When you've customised the object click build.

note that after you click Build (or before you click build) there is a Preview GameObject created temporarily.