How to use substratum on Android 10
1. Install Magisk.
2. Install substratum (not lite) from play store.
3. Install any Q-compatible theme.
4. Build your overlays. Depending on the Magisk version, substratum might crash after building your overlays, and that's ok.
5. Using any root file manager , go to your internal storage.
6. Enable hidden files, and you should see a folder called ".substratum", inside that folder should be all your overlays as apk files.
7.Move all of the contents of ". substratum" to system/product/overlay
8. Reboot and Enjoy
Note: You need to do this every time you update your overlays.