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.
A 6 by 4 inch print has a shape of 3 to 2. The shapes in the tool do not include that exact ratio, so choose Free and drag the box until the width shown is one and a half times the height. For example, 1500 by 1000 pixels or 3000 by 2000.
Leave a little room around the subject, because print shops sometimes trim a few millimetres at the edges. A photo of at least 1800 by 1200 pixels prints well at this size, and a smaller one may look soft.
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.
None of the presets are exactly 3 to 2, so use Free and make the width one and a half times the height, watching the pixel size shown as you drag.