Google Analytics for mobile application

Setting up GA4 for our mobile application. I have already partially setup GA4, but haven't setup GTM, and don't clearly understand how to create custom events. I am looking to setup custom events for my mobile application. If you have experience in GA4 for mobile phones, I will be happy to give you more details on what exactly I am looking for. We can discuss the scope of the project and then you can give me a custom offer.