By the sounds of it, the only part of the phone that’s damaged seems to be the screen so below I have linked the guide and replacement screen for the pixel 3a. The guide will tell you the tools you will need. You can buy them all desperately or you can buy the Essential Electronic tool kit, which includes everything you need. (Apart from the iOpener but I tend to find using a hairdryer to blast some heat onto it seems to work nicely as well if you don’t want to spend the extra money to buy one.
Google Pixel 3a Screen Replacement
Essential Electronics Toolkit