​Where do all those giant “Trusted-By” firms come from?

I’ve noticed that some websites list major companies as “sponsors,” while others call them “customers”, but mainly comes under the section “Trusted by”. In cases where a website/SaaS features well-known firms, how do these partnerships usually come about?

Are these big companies actually paying for a service, or is it more of a mutual promotion deal? How can a website establish such relationships with major firms? Would love to hear insights from anyone with experience in this area!