What if you used more than 1 Apple ID now you can’t find an app you bought a long time ago because it might be on the other I’d

It's up to you how much trouble you want to go through.

You could simply download the app with this ID.

You could set up Family Sharing with the other ID and then share the app to the new ID. To learn how to set up Family Sharing, read this support article --> Set up Family Sharing - Apple Support This presumes it can be shared, and if any in-app purchases can be shared.