How can I retrieve my Aperture Library from an external hard drive?

Assuming that library is intact ten the first thing you need to do is choose which app you're going to migrate to. How you proceed will depend on that choice. Some folks have gone to Photos, the freebie app that comes with your more recent machines. It's more equivalent to iPhoto than Aperture, though. Others have gone for more powerful apps like Lightroom Classic, CaptureOne, or OnOne Photo Raw. (There are other options.) Depending on which of these you choose the steps are different.

Photos can import images from an Aperture Library. Lightroom Classic can migrate an entire library. Using the Finder you can recover the original files - but that means losing all the work you've done on them. So, let us know how you want to proceed, and we can advise you further.