This is straightforward with browser addons like uBlock Origin where you can add and choose blocklists, but I did searches for doing so system wide and using a VPN but didn’t find clear answers. I could use a DNS service that provides blocklists but isn’t it best practice to leave DNS to the VPN provider? I looked up blocklists and VPNs but didn’t find relevant results.
On Android, I didn’t find any apps that let you filter blocklists and using your own VPN other than Rethink, but the blocklists feature requires using Rethink’s DNS.
So what’s the best way to filter ads and trackers on both 3rd party apps and on OS’s like Android (specific Samsung phones) while still using a VPN?
Why would it? The PiHole would be on your local network, so it would never need to go past your router. So the request itself would be private, what matters is what you do with the response. Theoretically, the PiHole would only give responses for things it’ll block (usually directing it to localhost or something), and have no response for everything else (check your configs).
So if you get a response from the PiHole, you route the request locally, which does nothing. If you don’t get one, you’ll check the secondary DNS, which is provided by the VPN service.
You should certainly confirm this before completely trusting it, but it should work fine.