Hi there,
they are the same thing ( kinda ).
So this is a common system font stack and the one that GP uses:
font-family: -apple-system,system-ui,BlinkMacSystemFont,"Segoe UI",Helvetica,Arial,sans-serif,"Apple Color Emoji","Segoe UI Emoji","Segoe UI Symbol"
In there we have a bunch of system ( default ) fonts.
When the site is visited the browser runs through the stack and chooses the first available font option on that device.
I run a mac so it will use the -apple-system
font which is currently San Fransisco.
On a Windows PC it will use the Segoe UI
So on all devices it will never need to download a font, as it will simply use whatever is available.