Extruding inwards properly

[Emaulligu]

Hey everyone, still pretty new to the software and facing a recurring issue which is causing me a bit of a headache and a good deal of frustration too.
I have my... cube, model, whatever you want to call it, and I'm trying to use the Extrude function on a face, and move it inwards. I know that Extrude Manifold normally takes the adjacent faces (took me a while to realise I could hold the handle then press Shift X at the same time because E only uses basic extrude). But sometimes, even Extrude Manifold doesn't take all faces. It leaves some faces behind, and when I try to delete these manually, it deletes the node in the middle of the face, but the face itself remains and I cannot select it anymore.
The only clunky workaround to this I've found is to delete the edges, which for some reaon deletes a huge chunk of other faces, then select the remainng edges and hit F to recreate a new face. Now this only works when the extra face is at top-level and not in the middle of the model, otherwise everything gets wrecked.

So I'd like to know what I'm doing wrong. Is there a special function for this I'm unaware of ? Is the problem in my workflow ?
I looked up videos on Extrusion, read the documentation, but I haven't been able to find a specific about the problem I'm having, so I would be really grateful if anyone could provide some help about it.

[The Renderer]

Your description of your problem is very confusing. Try again or better yet post a screenshot or two.

In general, extruding is equivalent to pushing the face inward *while creating new sides* along the way you push. Think pushing a rubber surface inwards. So if you extrude a single side of a cube (with "e") inward, it will push the side inward and create new faces going inside, so you get doubled sides.

Use it on a face of e.g. a UV sphere so you understand what it does and why doing the same on a simple cube leads to issues.

Extrude manifold is supposed to fix that automatically but maybe it can't do so in all circumstances. I never use it.

[Emaulligu]

Okay, I've grabbed some screenshots.

This is normal extruding, as expected it leaves all other faces untouched.
I've been using this for a while because I couldn't figure how to stack Extrude Manifold with the axis locking to make it use the 3D space instead of its own normals. Pressing keys while holding mouse 1 is really clunky, but anyways.
https://steamcommunity.com/sharedfiles/filedetails/?id=2988488281

This is Extrude Manifold. You can notice how the side faces are being affected too, as they should. However the top one is not.
https://steamcommunity.com/sharedfiles/filedetails/?id=2988488454

And here is my issue : I selected the extra topface that shouldn't be here, I pressed X and selected "delete face". And according to Blender, that face is now gone. Notice how it doesn't have the central dot to select the face, while the face I've extruded still does.
It's now impossible to select that face because according to Blender it doesn't exist anymore, yet the 3D model remains.
The only workaround I found is to select the edges of that face and delete them, which deletes the entire top plane, which supposedly is composed of two distinct faces.
And it's only really applicable because that top face is flat and there's nothing else on it, which won't always be the case.
https://steamcommunity.com/sharedfiles/filedetails/?id=2988488644

So I don't care how, maybe there's a tool I've completely missed in the options, maybe I've been doing everything backwards the entire time but I just need help to find a way for this to stop happening, I can't waste half an hour slamming my head against a wall everytime I want to make a simple extrusion.

[The Renderer]

Alright.

First, why the manifold tool leaves that upper side, I don't know, the tool is still pretty new (in comparison), maybe it's a little buggy.

But what happens afterwards I can explain: There aren't two faces next to each other, they are overlapping. The big face goes all the way to the left and covers the same space as the small one. Then you delete the small one, but the big one is still there and still covers the whole space. Then you delete that as well and the whole top (which at this point is only that face) is gone.

So that is working as intended, the best way to deal with it is deleting them both and creating a new face to fill the hole (also make sure to "m"erge by distance, just in case)


Now, how to keep that from happening: As long as Extrude Manifold does this, there is nothing much you can do. Maybe change your workflow to use it less?

[Emaulligu]

Thanks for the explanations.
However if the two faces are overlapping and if I delete the small one, where is there still a visible edge ? Shouldn't that edge be gone along the small face on deletion ?

I guess you're right though, I'll need to establish a better workflow for this all.
I'm still new to this, I have gotten a bit more familiar with the tools (at least the basic ones), but I don't really have a methodology associated with the process.
And while there's plenty of tutorials on how to use the tools, I haven't found something worthwhile on the process itself. Maybe I should look more into that for the time being.

[The Renderer]

That edge is not on the top face, it's where the vertical face that you pushed in ends.

If you don't understand what you are seeing (regarding what connects to what) you can always just select a side or vertex or face and move it around a little (and then undo), it makes things a lot clearer.

[Emaulligu]

Thanks.