https://en.wikipedia.org/wiki/Software_rot?wprov=sfti1 . Try reading this. If you are experiencing too many bugs try restoring phone using iTunes it usually fixes lot of issues. This due to upgrading phone directly on phone is not as clean as upgrading phone using external device which in this case is a computer which actually wipes the device and completely reinstalls os rather than upgrading existing os. Also I don’t think calling and threatening apple is what fixed his issue it’s probably placebo effect or he just simply restarted his phone which surprisingly fixes lot of issues not just phones but any kinda computer.