Facebook and Instagram ads drive a lot of first touches and a lot of debate about attribution. Ads Manager models conversions it can't observe directly, credits views as well as clicks, and can claim sales another channel closed. To see which ads actually sell, you need your own record of who clicked and who bought.
The foundation is tagging every ad with URL parameters that carry the campaign, ad set and ad ids, recording visits with a first-party tracker, and getting sales from your payment processor or store. Then send those real purchases back with the Conversions API, so Meta's algorithm optimises on sales that happened.