2015 MacBook Pro with Monterey’s latest update - start up screen altered

Is the icon you're referring like the soccer ball in this screenshot?

If it is give this a try: boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

NOTE 1: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.

If that doesn't clear up the problem go into your contact card in Contacts, remove and add back in the image you want to have like this:

Have you run Disk Utility's Disk First Aid on all connected drives?

In Disk Utility>View, select Show all Devices, highlight the top left entry.

Run Disk First Aid on all items in the left panel, from top down.

Are you running any VPN, Anti-Virus, or Cleaning apps?

We need to see what all is running, a report from this will not display any personal info...

Using EtreCheck - Apple Community

EtreCheck is a FREE simple little diagnostic tool to display the important details of your system configuration and allow you to copy that information to the Clipboard. It is meant to be used with Apple Support Communities to help people help you with your Mac. It will not display any personal info.

https://www.etrecheck.com/

Thanks for Old Toad’s etrecheck instructions…

Slow iMac 2017 - Apple Community

When you say " In the last hour I turned on my MacBook Pro (2015) and had an icon I did not set up myself'

Do you mean " User Picture " that you originally setup for your computer ?

Change User settings on Mac

Click the picture that appears next to your user name or another user’s name in the login window on your Mac.

You can choose a photo from your photo library, take a new photo using the camera on your Mac, choose a Memoji or emoji, use initials in a monogram, and more. When you choose a picture or other image for your user login picture, the selected image is also displayed for your Apple ID picture and for your My Card in Contacts. See Change a user’s login picture.