@Christina Paik, As a lifelong student, I have always loved the scene in the Wim Wenders film Wings of Desire in which the protagonists enter a library and everyone studying has an angel at his shoulder.

Get to a library, you’ll need the help.

Oak is very porous, but not evenly so, so even if you sanded this door to the nub, there’s no guarantee that your stain will penetrate evenly. I’m preparing you for blotchiness. Sand more deeply and try a pre-stain primer.

I’d do another stripping w your product. That milky brown area needs assessment ... another good stripping and see what is left of that. You may avail yourself of a good refinisher that can work w that once you’ve done your part. most Times I’ve done a stripping project its 2 or 3 times . It’s messy but once you are set up it can be quite successful as far as seeing what you have to work with . So much sanding is harder in a way and can be more traumatic To the piece.