SN is custodial. So we're all using SN's channels here and the cost of managing those channels is abstracted away from us.

A non-custodial wallet basically means you have your own channels, that nobody else can control. So if you spin up a Phoenix wallet and deposit funds into it for the first time, a channel needs to be created for you.

So kinda like this:

Receiver = Custodial -> No on-chain fees incurred Receiver = Non-Custodial with no channels / inbound liquidity -> On-chain fees incurred

Once the channel has been created, you can spend from it at little cost. If you want to create a new channel, change a channel's capacity, or close a channel, that all involves touching the base chain.

Savvy Lightning users will do their channel management in low-fee environments to minimize costs. Open a channel when fees are low, and spend from it when fees are high. But there will come a time where fees are always high, and most people will be priced out of opening their own channels (unless we see some crazy new developments in the space that I'm unaware of).