I want change the default font in my AOSP GSI builds. I have added them in /extrnal like the standard Roboto font. I have added them in /framworks/base/data/data/font.xml
But they are still not in the system.img
You can use the vendor_overlay system and add the .ttf prebuilt file and copy it to $(TARGET_COPY_OUT_SYSTEM)/fonts/ using PRODUCT_COPY_FILES. You will also need to modify the fonts.xml to include the changes and build the overlay package as well.
Please check these as a reference point from AOSIP Project since the answer is a bit broad:
https://github.com/AOSiP/platform_vendor_overlay/tree/eleven/fonts
https://github.com/AOSiP/platform_vendor_overlay/blob/eleven/fonts.mk