Hi,
root servers are used to prime the tld part, like "gime a server that knows about dot com ?!"
With the answer coming back, the Resolver will question that server, and ask where are the name servers of google.com ?!".
With that answer coming back, the Resolver will question one of these name servers of google.com, and ask : "gime the A or AAAA records of google.com ?!"
After all, these name servers of google.com are the only ones that the ones that can be trusted to answer that question.
root server do not cache every possible zone (domain info) of the planet earth. They couldn't do that.
So, yes, it's normal that you see many DNS servers being used.
VPN or not, "DNS" doesn't chance.
Example : ask if a root server - let's take 'a') knows the IPv4 of the domain forum.netgate.com :
dig @a.root-servers.net forum.netgate.com A +short
It can't ....
It will tell you where to find the guys that know all about dot com zones.
Etc.