Or more likely: Confirmation bias. I also occasionally get Nigerian memes on IG despite never having searched it. But unlike you I don't have the Nigerian film google search history that would lead me to conclude it was targeted.

Put another way, if you googled a Swedish film then saw Swedish memes you would think that was targeted. Then upon viewing the Nigerian memes you would have attributed it to a random video rather than anything targeted.

“ I looked it up on my laptop when I got home”

That is the real mechanism for triggering the ads and memes, not that “your phone is listening to you.”

At least feel creepy about the real cause and not some imagined monster under the bed.