If it is a product from a huge/multinational company then you can guarantee the product launch is supported by an all-out assault on all marketing channels. Price promotions and product positioning (within your eyeline) coerce you into trying it and then tv ads, social media etc build up understanding of the product and tell you this is something to trust and support. It can seem like a strange coincidence if you just bought that product and it is now on your social media feed, but that is what happens to many people at the beginning of a product launch.
Was it Bud Light by any chance? I have always preferred strong beers and watched American TV thinking how pointless a beer that weak could be (just have a tasty soft drink if you don’t fancy a beer!), and now that it’s hit the UK it is all over the shelves, on TV, social, radio - everywhere! I cannot get it out of my face.
I don’t think Monzo have bought your data. They are building a brand based on transparency, they are also pretty much pre-revenue and have a very simple early business model so I don’t think they would dabble in these dark arts to make money.
Whilst dismissing Monzo as the culprit, I definitely think there are unscrupulous techs out there who do use underhand and creepy methods to sell you advertising. Until recently my other half had a Samsung S5. When she ran Facebook it would come up with adverts about whatever we had just talked about. e.g. if you said “i’d love to go to Disneyland” and load up facebook then hey presto - targeted ad from Disneyland. Talk about crisps and hey presto Walkers are on your feed.
Many people have accused Facebook of this and they publicly deny they use your background sound to target ads at you. Having played around with that phone I am highly doubtful.