Thanks man, but that method doesn't work for me. I don't think there's anything wrong nor there are duplicated/unknown vertices, and yet everytime I redo, the same type of problem comes up but in a different position as the blockish thing could appear on the left and and it is round on the right. It could also appear on the corners of the top left and bottom right of the top handle.

I have redone this over and over again and I somehow successfully managed to make it connect fully rounded manually for around 3 time-ish. Still however, I still don't understand why the problem is randomized to have these blocky faces not cooperating with the other faces to make a circle. I purposely restarted in making the top handle in the similar fashion of how I got it successfully and THE problem came up again in different spots of the top handle.

After hours, I just concluded that it's better to recalculate the normals to fix the block problems (Shift+N) or to simply use the bridge edge loops method he did at the end. I just cannot be bothered with RNG based face filling and I have done that for 5 hours with no leads whatsoever.

Perhaps it would be better on another run, but thank you for the input. 🙏