Sadly its very poorly documented and hard to find info around. My best suggestion would be to look how open dota fetches it. The project is on github.
Theres also the valve wiki https://developer.valvesoftware.com/wiki/Steam_Web_API you'll have to do some digging there to figure out which interfaces you need to call.
(https://www.reddit.com/r/DotA2/comments/wymkag/dota_2_official_api/)
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Maybe somebody here knows where can i find valve`s API for dota 2, i have been using opendota API but it doesn`t have all data