Ingonimania
In order to provide answers to your questions, I will be quoting the GrapheneOS FAQ.
Does that mean that in my MAIN profile the traffic goes thought the private DNS and in the DAILY profile the traffic run throught private DNS & VPN?
Yes. Private DNS takes precedence over VPN-provided DNS.
I read a lot that its better to choose either one or the other...
If you're using a VPN, we recommended against having a Private DNS server configured. If you want to filter traffic while using a VPN, use a VPN service app able to do both such as RethinkDNS. Private DNS also interacts strangely with multiple profiles since each profile has their own VPN configuration but Private DNS is global. Either leave Private DNS on the default Automatic mode or set it to disabled when using VPNs.
WHATS THE BEST WAY TO USE THEM WITHOUT GETTING EXPOSED?
Using the network-provided DNS servers is the best way to blend in with other users. Network and web sites can fingerprint and track users based on a non-default DNS configuration. Our recommendation for general purpose usage is to use the network-provided DNS servers.
Apps and web sites can detect the configured DNS servers by generating random subdomains resolved by querying their authoritative DNS server. This can be used as part of fingerprinting users. If you're using a VPN, you should consider using the standard DNS service provided by the VPN service to avoid standing out from other users.