if i changed my DNS (mmm let’s say to dns.watch) my ISP will see my traffic ?
Yes, in fact they’d even still see your DNS traffic
Yes, as Jonah said, your ISP would also see your DNS traffic, because DNS by default is not encrypted unless you are using DNSCrypt, DNS over HTTPS (which looks like HTTPS) or DNS over TLS.
Even if you were using those, your ISP would still see your other traffic, unless you were using Tor (in which case they would see you are using a Tor in case you were using a Tor bridge) or a VPN (in which case they would see you using a VPN).
Your traffic would include the details where you are connecting to and in case of HTTPS also the domain you are accessing due to Server Name Indication which tells the server in plain text which domain you want so the server knows to give you the correct certificate in case it hosted multiple websites. There is also encrypted SNI, but it’s not widely supported yet and it would mostly only hide the website you visited if it used Cloudflare or a similar CDN offering multiple websites on the same IP address(es).
We have an GitHub issue about this,