I'm a medical student and not sure how to start with building my app?

Hi guys, I'm a medical student and I'm not really sure how to proceed in how to build my app. I know this is a long post, so if you have some time and would be so nice to help me by providing some comments or views, that would be really appreciated.<br><br>I'm a medical student with some coding skills. Last year I built an app that will help you memorize anatomy using flashcards. I built it for myself, my colleagues love it and I've shared it to some online platforms and many people have downloaded it, so I decided to continue working on it and expand it. The idea is to have an app that you can use on your phone for studying anatomy and to help you student. It has all the human anatomy, with some definitions, locations and you can search and mark the muscle, organ, bone, system, etc. I know there are many apps like this already (Complete Anatomy, Kenhub, etc.) but I want this one to be more simple with less ads, if any, and also to be free.<br><br>So far I have built the app for android and iOS and I really want to monetize it in some way and have the app sustain itself. So my idea is, to make it subscription based. You can either buy a monthly or yearly subscription and have no ads, or you can pay a one time fee for the premium version which will have no ads and will be ad free forever. I currently use the revenue stream from admob to finance the app, but I want to migrate to Google Play Billing.<br><br>My current problems are:<br><br>1. How to implement the subscription logic in the app? I have built the UI, but I'm not sure how to tell the app that a user has purchased a subscription, and they should no longer have ads displayed. I know that using Google Play Billing you have to use APIs and other stuff, but how to know when they buy a subscription and when it should be applied?<br><br>2. How to know that a user has bought the one time fee premium version? If you buy the premium version, I'm thinking of showing a message saying "Thank you for purchasing premium, you will no longer have ads!" and I want to display this only once, when they buy it, but I'm not sure how to persist this and save this info for when they start the app.<br><br>3. Should I save this info on a local database or on my server? What if a user reinstalls the app? If I save it on a local database, when they reinstall it, it will be gone, but if they register an account and login when they reinstall it, I can save this info on the server and it will be ok for when they reinstall, but what if they reinstall it and don't have an internet connection?<br><br>4. What if a user reinstalls the app on a different device? If a user reinstalls the app on a different device, it will be recognized as a different user, and they will have to buy the premium again. I know that some apps, like Spotify and Netflix, recognize that a user is logged in on another device, but how do they do that? Do they use some kind of ID? If a user logs in on a different device, it will get a different ID, but how do they match it with the user's account?<br><br>5. How to deal with the logic that a user buys a subscription on another device? If a user buys a subscription on their phone, but they have the app installed on their tablet, it should be recognized that they already have a subscription and it should be applied? How to deal with this?<br><br>6. How to ensure that a user that buys a subscription will not get their subscription removed even if they reinstall the app? As I said earlier, if a user reinstalls the app, how to ensure that they will still have their subscription?<br><br>I know that I should probably use some kind of database, but what type of database? Firebase? What is the best solution? Again, this is a long post, and I would be really happy if you could help me out. I'm trying to release the app to production and this is my last step before releasing it.<br><br>&#x200B;<br><br>EDIT: I've had many comments recommending me to use Firebase, so I will probably try to do that. Thank you really much guys!