Good day, if you want to access files from an unbootable Mac, iBoysoft Data Recovery Software is highly recommended. iBoysoft Data Recovery, taking advanced and safe techniques, helps recover data from an unbootable Mac in an easy way. It can be launched in macOS Recovery mode through Terminal without creating a bootable drive.
Read this article to know how to retrieve the files from the Mac that won't boot to make a backup: How to run iBoysoft Data Recovery in macOS Recovery mode