DNS is hierarchical. If your DNS server manages example.com then you can set a delegation for a subdomain - e.g. www.example.com - to someone else. I.e. you could say "for this subdomain the nameservers from google's cloud service are responsible". You can go further and e.g. configure the google cloud service so that abc.www.example.com goes yet to another DNS service.
So a company could say "we manage the DNS for our domain inhouse, but for www. we let some other company do it and pay them for it". This is pretty standard.
DNS is hierarchical. If your DNS server manages example.com then you can set a delegation for a subdomain - e.g. www.example.com - to someone else. I.e. you could say "for this subdomain the nameservers from google's cloud service are responsible". You can go further and e.g. configure the google cloud service so that abc.www.example.com goes yet to another DNS service.
So a company could say "we manage the DNS for our domain inhouse, but for www. we let some other company do it and pay them for it". This is pretty standard.