In the video he said to make the base a little shorter by selecting the base face, pressing G then Z, and move it up, it worked on the video, but when I tried it only the face moved up and inside the lighthouse and everything else stayed in the same position… I tried also selecting everything at the same time to no avail
It seems a stupid thing to get stuck on, so any help would be greatly appreciated
Not at all; it took me some getting used to as well. Grant goes into a little more detail with this sort of thing in the dungeon section of the course.
I’m pretty sure what’s happened is you have duplicate topology left over from cancelled extrusions. Extruding is well-known for causing all sorts of problems for beginners because RMB doesn’t cancel it the way it cancels other things. When you use RMB to cancel after pressing E to extrude, all it does is cancel the movement you would be making, but the actual extrusion still exists (and now sits directly on top of whatever you extruded from). To cancel an extrusion cleanly, RMB to cancel the movement, and then press Ctrl + Z to undo the extrusion itself.
To fix the duplicate topology you’re now left with, in Edit Mode, press A to select all, M–>By Distance (btw, when you see topology weirdness in general, this is a great first step to try). This will merge any duplicate vertices (and by extension, any duplicate edges and faces). Then you should be able to shorten the base like the video shows. Have fun!