I replaced the screen on my daughters cheap Android phone, first time I used double sided sticky tape, you can buy it specifically for the job in sizes down to 1mm wide, but this was useless and come unstuck, although it did come in useful for other repairs.
Second time I used B-7000 glue which has lots of recommendations, it is recommended to leave it to cure for 48 hours though. This stuck the screen well, until she broke it again, although it was still usable.
I've also replaced the battery in my Elephone P9000 which was pretty straight forward.
I've no idea how easy it is to change an iphone screen/glass, with my daughters it involved warming the screen with a hair dryer to soften the glue and then some delicate removal of leads. I did try replacing the glass on a Samsung tablet that was broken, but that ended badly.
One thing I have learned is they are very fiddly to repair, you need a good steady hand and plenty of light/good eyesight.
PS. Just about to pull my phone apart as the loudspeaker has stopped working - have ordered a replacement from China for £3.71, but just want to check a wire hasn't come off. This will really test me as the speaker wires are soldered on