Where is the registry settings for the Task Manager 'Startup' folder

Under Windows 7, at least, there were no Registry keys associated with individual items located in either Startup folder - not the Windows Startup folder nor individual users' Startup folders.

If you have psexec deployed and the offending item is in the Windows Startup folder, you might be able to delete the shortcut remotely from

ProgramData\Microsoft\Windows\Start Menu\Programs\Startup

There are many regkeys actually, even from WinXP on. In Win10, most are still valid, and will arrive in Task Manager all the same.
So it won't help you that much just looking there.
If you really must know in full detail, use this MS tool instead: https://docs.microsoft.com/en-us/sysinternals/downloads/autoruns

The registry keys, if they exist, should have an optjon to enable or disable that.  Because that is is how it is done from Task Manager --> Start up tab --> right-click that desired program --> select to disable or enable it.

Just like in : https://www.techrepublic.com/article/get-more-information-on-window-10s-startup-tab-by-enabling-additional-columns/

I have already found HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Run , and I can manually delete the files there but I will have to do this at login every time.  That will be a backup plan.  I want to try and find a reg. key that I may disable instead of delete.  http://eskonr.com/2018/11/how-to-disable-or-enable-auto-start-of-teams-application-using-gpo/

Well for a software program to be installed enterprise wide, I can see some users being annoyed that the launch screen is coming up automatically.  It would be nice to disable that for them ahead of time; but, with this user profile setup structure it does make disabling that launch more difficult.

Even if I have a task scheduled to run at "login".  It will be off to the races to see if that task will delete the registry key before MS teams attempts to launch it.   Perhaps having this registry delete task run every 5 hours intervals may help make sure the auto launch will be removed before morning?  User settings registry removal is more problematic. 

Thank you for your efforts.

Scheduled tasks also has the  option for a computer start. But yes, for user based startups, you'll miss one startup, as even the GPO will kick in too late. But after that, it will never auto start again. Removing the key at logoff or shutdown will also ensure next time will be clear of the start up program you removed.