My guess would be your Windows ID. Did you use the same Microsoft account when setting up the new laptop as your old one? I know Brave should eliminate that but from what I've heard it just doesn't. It's all lies. You mentioned using Edge, and I am thinking that Microsoft and Google are both so heavily intertwined at this point that they are basically the same data cesspool.

Of course that is assuming it was a PC and not an Apple or Linux based device (again, you mentioned Edge). Other options would be a Samsung, HP, Acer, Google, or other manufacturer accounts. Google for sure. If you signed into Google anywhere else on that thing with your ID I would just assume your data was being beamed straight into every computer watching for it in existence.