What exactly happens when you attempt to boot the system?
Any error messages?
Run the third party app EtreCheck and post the complete report here so we can examine it for possible clues.
How to use the Add Text Feature When Posting an EtreCheck Report - Apple Community
Check the "/Library/Logs/DiagnosticReports" folder and even the "Retired" subfolder to see if there are any Kernel Panics logs. Kernel Panic logs usually have file names containing "kernel" and/or "panic". Post several Kernel Panic logs here using the "Additional Text" icon which looks like a piece of paper on the forum editing toolbar.
While the EtreCheck report will include summaries of the Kernel Panic logs, sometimes I find that summary does not always capture the critical part of the log, plus macOS will quickly delete some of these logs.
Was the Apple tech able to see the issue first hand?