Honestly, don't handle that yourself, it's too tiring if you are a solo developer and want to keep things simple and cheap. Just use RevenueCat (apparently it is forbidden to link to this website... what a joke). It's free until you reach 10k$/month and they have a great tutorial.

I have the download button but it doesn't download anything (Firefox and Safari).

Having this issue on iOS 14 and 15 based on insights from Firebase Crashlytics. The only common point I found between users is that they have low RAM memory... No idea how to avoid this issue.

It's working now! 7 builds suddenly appeared and some of them were still processing. My guess is that some of their servers' task queue was jammed.

Same issue here. Several successful build uploads, no email received about any issue. It has been a day and the builds are not visible at all on AppStore Connect. I have reached Apple support, waiting for a reply.