How to add Expansion addons to app. (Android)

I'm a new app developer and I know the least about Android. I've developed a Notepad.

I want to keep its installation size minimal and I would like to add options like fonts as an optional downloadable addon.

I do not have any code or logic of how to make that happen, but since I have seen this type of support in a few Android editing apps, it's definitely possible.

I need a full tutorial to help me do that in my application. Kindly help me out of it.

My Notepad is a simple Note making app based on the tutorials provided by Google and uses SQLite database, it is heavily focused on themes and looks and has a very beautiful note sharing system.

You can check my app at playstore named as "Easy Pad" by Reliquary.