Can an ISP still see what URLs one visits when using a VPN?

if it's correctly set up, all your traffic will get through the VPN and your ISP won't be able to see anything.

Split tunneling allows regular traffic to go directly via your ISP to process those requests, and any VPN traffic would go through the tunnel and is encrypted. Your ISP can only see that you have a connection established to a certain address, but not the content of traffic going through it.

If you want all your traffic to be encrypted, you force all network traffic to use the tunnel. Latency would be higher in this situation.

You can test this by checking your WAN IP address and see if it differs from your ISP and using a DNS leak test, to see what servers are serving your DNS queries or even doing packet traces.