a yellow spark/lightning bolt randomly shows up on my screen. I think it means that the Pi is not getting enough amps from the PSU. My Pi also keeps rebooting randomly. I'm currently using a Microsoft phone charger to power my Pi, but I also have a better PSU that outputs 5.2V and 2 Amps. Can I use this PSU to power my Pi?
No! It DOES NOT mean you are getting insufficient current! The voltage is too low. You can have a PSU which can supply 12A, but if it can't do this at an adequate voltage you will have problems.
Your phone charger may or may not be OK, but many chargers DO NOT provide 5V (they are designed to charge 3.6V batteries). Even for those that do, often the cables are often inadequate.