Crypto movement tracking app, advice on how to build one

the basic app idea is i would like to track the movements of crypto currencies when they reach a certain point when they down a certain percentage and so forth, i know a lot of apps already do this but i would like to know how i can build my own that way it is fully customizable and its displayed the way i want, please let me know if there are any open source git hub repos that i can look at and get an idea or any API's that exist that already pull this type of data so i can easily implement it, i don't mind the tech stack whichever is available. Also I would like to know if they are any legal walls that come with building something like this. I would not like to add any linking of wallets and accounts I'd simply like push notifications through emails so i can use the already certified ways of trading crypto.

grammar is poor please excuse