If you like to have a very smooth environment you should put a very big value in the fillet slot. Now we need to create an outline of this shape (the thickness of the walls). Select "spline" and go to outline the spline with the command "outline" that you can find again in the modify panel.

From this point you can create both environment objects (the linear or the circular one). To create the "straight" one (which will be the one that I will use for the tutorial) you must assign to the spline an "extrude" modifier from the modifier list.


To create the "circular” environment object you must first move the PIVOT/GIZMO to the correct location.



Just go to hierarchy, click on the button "affect pivot only" and move the pivot (this button allows you to move/rotate the pivot independently from the object)! After this step assign to the spline a "lathe" modifier from the modifier list. You will see that you will create an object similar to a tube. Go to the modify panel and increase the segment number to have a smoother shape. Now go to Degrees and put 180° in the slot. You should have the result of the image below.