Best Practice For Font Use in Mobile App Design

I'm currently designing an app using Figma and having had no prior experience am learning as I go.

I appreciate both Android and iOS have their own design guidelines which I'll be looking at, but I was hoping somebody could provide advice on font.

My question is - is it typical for an app to use a different system specific font for each platform? i.e. - using Roboto for your Android app and San Fransisco for your iOS app?

Or is it more typical for apps to use a custom or paid for font such as Helvetica? In the case of using a custom font or paid font I guess it negates the need to use two separate fonts for each platform as providing continuity across platforms.