It could a myriad of things, but Meta knows a lot about us from our past behaviors, interests, connections, etc. Even if you're totally locked down in the woods for about a year, and log in to your account on Vanadium you will still have relevant ads.
For example, your family and friends might post a photo including you at a concert. Meta will now show you ads about a nearby concert, in your city! Even the photo your friends took was blurry or you're in the corner of the shot.
Your close friends searches for a car. Since you are close friends, you might have similar interests, and now your feed is full of car ads. And weren't you just talking about cars yesterday?
VPN is useless in this case
No Meta apps on your phone is useless
GrapheneOS will not prevent relevant ads from showing. No matter how careful you are, you're circle will betray you and leak info.
You could however, turn off web tracking in the settings. Meta won't show relevant ads, but they still know you.