Micropayment or Play Store Link (Help!)

Hi everyone, I have created a free version of my puzzle game with 25 levels and would like some help, if possible!

I would like to link the user after they have finished the 25 levels to the complete game of 100 levels in the google play store or have a micropayment to download just the specific files with the extra levels.

I don't expect to get a lesson here about how to do it, I just would like to someone to point it out where I can find information on this topic or some nice tutorials about how to do this with unity because I'm really clueless and don't even know where to start looking.

HELP?!?

Gracias and Many Thanks!