There are so many previous posts about this exact same issue… If you search for ‘broken mesh’ or ‘faces missing from mesh’ or ‘mesh not the same’ or ‘parts of mesh disappeared’ you’ll find similar posts.
You have your Normals reversed.
Google ‘Blender show normals’ because there are a few ways to display which side of the faces are pointing out of the mesh and which point inwards which makes them invisible from the outside.
Someone is probably going to tell you to click the Doublesided checkbox, but don’t. You’ve got a broken mesh and Doublesided is not the way to fix it.
If you read my post you should probably have noticed that I mentioned that I’m new to blender, how would I know that my normals are reversed if I have little to no experience on blender?
I also mentioned that I tried various posts before actually posting for help since I know answering an already answered question is annoying, however, from the solutions I tried, none worked, perhaps I searched in the wrong place since I don’t know what a normal is and I don’t know why mine are reversed
Now I got it to work after googling how to fix reversed normals
Anyway, you mentioned Doublesided is not the way to fix it, yet, you’re not telling me which is the right way to do so.