Can my ISP monitor my network activity if I use nextDNS private DNS??

NextDNS doesn't encrypt everything, only the DNS queries as long as you don't use DNS via IPv4 or IPv6 but encrypted DNS (DoH, DoT or DoQ). After your device ask NextDNS what is the IP of x address and NextDNS send the answer NextDNS (or any other DNS) does not have anything to do with the  internet traffic.

The ISP can see what are you doing on the internet if you use their DNS and if you use another DNS then they can look at the connections you do.

To hide from them you need to use encrypted DNS from  someone else and you must use VPN, or Tor (Orbot) or Psiphon etc.

Most of the time your ISP still sees what domain you're visiting, because even with encrypting the DNS, the actual connection is either unencrypted HTTP (very rare today) that still includes the destination domain or HTTPS without ECH that still exposes the domain in unencrypted form inside SNI. ECH will remove that weakness but your ISP will still see the destination IP and unless the website uses shared hosting or CDN, it's also trivial to either compare against the existing list or just send a handshake directly to the IP since usually, the server will still reply with a cert that corresponds to what domain is served.

Public tracker's torrent can still be detected by copyright owners (your ISP doesn't actively monitor your torrent, they just forward copyright owners' DMCA notice) if that's what you're doing.

To actually encrypt everything, you need an actual VPN, note that your ISP can still guess if you're torrenting (large traffic in both directions), streaming (medium, intermittent traffic at long periods), downloading (one direction maxing the bandwidth) and browsing (small, intermittent traffic) though not what destination.