Photos from different cameras and different phones arrive at different sizes. Making them match is a single batch operation.
Resizing keeps each photo's proportions, so setting a common width leaves portrait pictures taller than landscape ones. For a tidy grid you need a common shape first, which means cropping, and only then a common size.
Crop then resize, in that order. Cropping decides what is in the picture and resizing decides how many pixels it has. Doing it the other way round means cropping an already small image and throwing away detail you cannot get back.
Pick your target from the largest use you have in mind. Scaling down looks fine, scaling up does not, so it is better to start too big than too small.
Crop to the shape you want first, then resize. That gives exact dimensions without squashing anything.
Yes. Enlarging cannot add detail that was never captured. Aim for the largest source you have.
No.