Idk why a triangle type thing forms when I fill this last part can someone please help
not seeing verts attached to it, so my first guess would be a modifier is causing it.
Hi Hibah HHibah ,
there is also something at the back:
I think that you had some extra, unwanted Vertices Selected when you tried that Fill.
Delete the Selected Face, then, for good measure, Select all (with A) and M > Merge > By Distance. Immediately followed by SHIFT+N.
Also make sure, that all Vertices that need to be Filled lie in one plane, because they don't look like they do.
Then Select the correct Vertices and triple-check that you don't have any other Vertices Selected and press F to Fill.
Hope that helps.