I don't know how well it works in practice. The other day I bought something from a local webshop (bike parts) on my laptop. The next day I'm seeing an ad from the same webshop on my Facebook feed on my mobile. Yes, it could be coincidental though I do see a lot of bike-related ads and practically never this company. (Even though I am a returning, if not very frequent customer of theirs.)

Some companies upload their customer data to FB because Zuck promised them then you can send ads to your existing customers.

I made the mistake of having the same email address for Facebook as for registering to random online shops...

I use a separate email address basically everywhere so this can't be the reason. (I don't even use my main email address for facebook.)