If the ad is under a filtered domain, it will simply not load.
If the ad is under the same domain as the site you are using (ex. Youtube) they will load just fine.
When the primary DNS is down, the secondary DNS will be used. This is the same regardless if PiHole is used or not, but is how DNS works.
If the ad is under a filtered domain, it will simply not load. If the ad is under the same domain as the site you are using (ex. Youtube) they will load just fine.
When the primary DNS is down, the secondary DNS will be used. This is the same regardless if PiHole is used or not, but is how DNS works.