How does one can track a link to sale conversion ? [Question]

Hi, I'm wondering how does a system to track someone from the moment they clock a link to the moment the product advertised is made ?

Tracking a click, add utm to get some metrics and shorten that: I grasp the concept

But shall the utm be present in the URL at all moment to know the conversion funnel ?
What if the website adding theses utm decide to lose them on the buying point ? Or simply not returning the data that an item was bought via the sponsored link ?

Thank you in advance for your answers !