Using LR Catalog files copy to open already adapted LR photo's from external HDD

As far as I understand you right you like to have a second HDD on which the corresponding LR files (catalog, etc.) are synchronized 1:1 and kept up to date. You need special software which is done these synchronisation. I don't know if such software is available for MacOS and if yes, it's working with Lightroom.

The easiest way is to make regularly backups. So you can restore your work in case of a crash.

How can I install or set, my Mac computer, external HDD so that if my original HDD crasches that my szecond backup HDD will startup Lightroom, and that he recognise the adapted files, without re installing and starting from scratch.

This has nothing to do with setting up your Mac. This is all done in Lightroom Classic. You point LrC to the second backup HDD where the copies of the files are located. Instructions

Also, the photo files themselves are NOT adapted or edited, they remain unedited on your disks. All your work is in the LrC catalog file(s), which if I am reading you properly, you have not backed up. You need to immediately, right now, no excuses, make sure you have backups of all of your catalog files on a different physical disk than the originals, and that these backups are updated regularly.