Apps opening at startup with no apparent reason.

You do gave 2 Login Items:

User Login Items:

[Not Loaded] StartUpHelper (Spotify - installed 2020-01-18)

Modern Login Item

/Applications/Spotify.app/Contents/Library/LoginItems/StartUpHelper.app

[Not Loaded] WhatsApp Login Helper (App Store - installed 2020-02-02)

Modern Login Item

/Applications/WhatsApp.app/Contents/Library/LoginItems/WhatsApp Login Helper.app

Go to System Preferences > Users & Groups > Login Items and select each and use the (-) button to remove them. Then restart and test.

Maybe the computer is not shutting down properly in which case it may relaunch the apps to their former state regardless of the "Reopen Windows" checkbox being unchecked.

Run EtreCheck and post the report here using the "Additional Text" icon which looks like a piece of paper. If you allow EtreCheck "Full Disk Access" the report will also include summaries of the most recent system logs which may provide clues as well.

Spotify can launch itself at start if checked to do so. Within the Spotify app, go to Preferences and make sure that the setting to open at startup is turned off.

You can check other startup items by going to System Preferences, then Users & Groups, then Login Items.

It's a bug in Catalina. I get the same thing even though there are no login items, and all check boxes to prevent apps from opening on a startup or restart are set correctly - the same way I've had them since this iPad like behavior was introduced in Lion.

I can have Safari, Outlook, etc. all closed before I shut down, and they will still randomly launch themselves on startup.

I've filed a bug report to Apple on it. Whether or not they consider it important enough to investigate determines when, or if it ever gets fixed. Just like the handful of fonts installed by Apple that will not show up in any of their own apps since El Capitan, unless you replace them with the same named versions from (specifically) Yosemite.

From the replies of other users, it doesn't seem to affect everyone, even though you'd think it would. For me, I can startup cold, or restart my Mac five, seven times and get what I expect - no apps opening on their own. And then the next time, the last apps I closed will launch themselves.

To report bugs, the link is one of the defaults in Safari:

You don't need to be a developer to sign in and report a bug, just your normal Apple ID and password.

Be aware the engineers expect at least a reasonably accurate and useful bug report with as much detail as possible. Not, "It doesn't work." or "I've tried everything."

You have two bits of software designated to run at login (WhatsApp and Spotify). To see if it is something like that causing yoiur problem, you could try starting up in Safe Mode.