Print sizes and camera sizes are different shapes, so something always gets trimmed. Choosing what gets trimmed is better than letting the machine choose.
Most phone photos are 4:3 and most compact cameras shoot 3:2. Standard prints come in several ratios and none of them match everything, so the lab crops automatically, usually from the centre, and usually without telling you.
That automatic crop is where heads get cut off and where the person at the edge of a group photo disappears. Cropping deliberately beforehand takes a minute and removes the surprise entirely.
Resolution matters far more in print than on screen. A picture that looks fine on a phone can be visibly soft at 6 by 4 inches, so use the largest original you have and do not resize it down before printing.
3:2. A 7 by 5 is 7:5 and a 10 by 8 is 5:4, which is why one photo rarely suits all three.
Cropping removes pixels from the edges, so a heavy crop leaves less resolution. A modest crop of a modern phone photo is fine at ordinary print sizes.
No.