In Android there are basically three kinds of fonts (a/k/a Typeface):
- Default fonts: DEFAULT, DEFAULT_BOLD, MONOSPACE, SANS_SERIF, SERIF
- System fonts: Anything in /system/fonts and referenced in /system/etc/fonts.xml
- Anything else: In a non-standard location
Some apps can only use 1, or 1/2 or 1/2/3.
Worst case: If you are using Magisk you can insert fonts into /system/fonts and update /system/etc/fonts.xml if your app is 1/2.
Does your app give you a choice of "NotoSansDevanagari"? Then it's a 1/2.
Sorry, I don't use Onyx apps.