Hello There!
Not sure why Grant is creating the insert for the turret the way he is doing it. I found it much easier to use extrude + scale inwards. And to maintain the bottom surfaces at the same level, I re-positioned the 3D Cursor and set transformation to 3D Cursor.
I made a short video >>>
(low FPS - Blender and OBS decided not to cooperate today)
Cheers,
Ray
Nice. I suppose it is the many ways of Blender, and Grant thinks the way he did it was easier for beginners.
Both his way and yours gets uneven recess depths top and sides. But easily fixable.
Think I might have done it the way he showed as trickier, as rearranging those verts I do not find a problem.
True - there are many ways in Blender to get the same result.
Not sure what you mean by “uneven recess depths”? The bottom surface is flush. Or do you mean the difference in depth between the top and the sides? Didn’t pay attention to this for this example. Good point.
Playing around
However, this happened and is a bit concerning. And Merge by distance doesn’t fix this.
When moving the highlighted edge, there is a face left and right to it. I didn’t expect this. The CAD software I used to work with would take care of a zero area face and remove it. (I know, I know… this is Blender 
)
I dissolved the bottom edge of both triangles with Ctrl + X. When I move the edge now, this looks as it should.
Lastly with snap to vertex + move in Z the face is at the right level.
Out of interest, is there a setting to prevent this from happening? Maybe @Grant_Abbitt has a thought about this? (Summoning the master)
Cheers,
Ray
PS: Going back to my triple gun truck and fix this now.
Yes
It is interesting there is a new addition to Extrude in Blender 4. Extrude manifold, but it can not cope with this. Well I have not made it do so, so far.
Easiest way.
As you do set the cursor on the base and as pivot point.
Extrude in the three faces.
Select the 4 edges in image one and delete them.
Select base edge loop now left to F remake a base face.
Done.
Interesting. Kind of works - but only at one axis at a time.
And when extruding the top face inwards, it created an n-gon (at least that is how I understand this right now).
Will play more around with this 
Why not use grid fill, to have quads?
Extrude manifold is designed to solve this problem, on the right it was extruded manifold.
Grid fill. Did not think of it just a flat base plate, I may have joined across if I felt like it if doing something for real not experimenting.
