Actually in this case it's DNS hijacking, not DNS cache poisoning.
The difference between DNS hijacking and DNS poisoning is who the responder of the DNS request is.
In the case of DNS hijacking, your machine makes a request to an upstream DNS provider asking "where is www.google.com" and it responds "www.google.com is at 18.104.22.168" without bothering to find out where it actually lives. This is in contrast with a normal DNS request where the DNS server would either query the real address from its cache or an upstream provider and then tell you with whom you need to connect to reach www.google.com.
DNS cache poisoning is where someone else's machine sends a request to your upstream provider asking "where is www.google.com". When that machine requests www.google.com from its upstream provider, the attacker then tries to "race" the DNS response. So the poisoner effectively asks "where is www.google.com" and then throws lots of "www.google.com is at 22.214.171.124" at your upstream DNS provider. If one of them "sticks", your upstream DNS will think that www.google.com is at 126.96.36.199 and will cache the response.
Now, when you make a request to your upstream DNS provider, instead of it asking its upstream provider, it "knows" (i.e. has cached) that www.google.com is at 188.8.131.52 - a domain owned by the attacker.
But in your particular case, you're not being attacked. Your ISP is deliberately misreporting where www.google.com is in order to redirect you to an internet gateway portal, rather than to steal your private information or hijack your cookies. Alternatively (since we're talking about youtube), your ISP might have an agreement with Google to host some content geographically closer to you to reduce the load on the ISP, which might require your ISP to "spoof" youtube to a false address within the ISP network.
So in summary, you're being DNS hijacked, but almost certainly not maliciously.