Put the phone in Safe Mode which disables your downloaded apps and leaves the stock ones working.
Your stock apps i.e. the ones already on the phone out of the box wont have ads built in to them.
If the ads stop in Safe Mode then come out of Safe Mode and start deleting your most recent apps one at a time until it stops as one or more of your apps have built in ads.
Current Phone ~Samsung Note 20 Ultra 5G SM-N9860_256Gb SnapDragon 865+ Samsung One Ui 3.1/ Android 11.