A nice and simple way to determine what process occupies a port in Linux is using lsof
For example if you want to see what process is occupying port 3000.
❯ lsof -i tcp:3000
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
grafana-s 2782596 gvoina 14u IPv6 32302690 0t0 TCP *:hbci (LISTEN)
Looks like we have here a Grafana server working in background.