Conditional Access: Can someone please explain sign-in frequency and persistent browser session

Persistent session applies to "all apps" because the browser "shares" the cookie with all resources it applies to, you don't get a separate cookie per app/resource. It's not directly related to the sign-in frequency in the sense that it's a simple "on/off" switch, either there is a cookie after you reopen the browser or not. But it does have a validity too, so you cannot use it indefinitely, so in that sense it is also tied to the SIF window.
For shared devices, you'd probably want a separate policy.