Game Making Method , Levels into one project ?

Unity Engine
1

hi
i have a q
for example in my game i have 3 level or 3 Terrain. what do i must to do ? insert 3 level in one project or split levels to projects and load one after another ?
sorry for English.
how i must make my game ? 1 project and 3 levels included or 3 project that every project has a level ?
did you understand my mean ?

and another Q : can i say every scene is as a level ? and make 3 scene and load it ?

2

Save your levels as individual scenes with unique names in your Unity project folder,
each one must be selected in the Build Menu when you build your game.
Then, move to the levels from the starting level by scripting,
by either colliding with an object with a load level script on it or some other way such as completing a task.