I've got a collection named contactData, inside this collection I have 100 documents.
Inside my flutter application I'm generating 4 random numbers from 0 to 100. Afterward I'm trying to read from my collection documents at these indexes. But it seems that I can refer only to the document name.
Is there a way to grab the documents by it's index in collection without providing index inside each document?
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Firestore queries are not index based, as that just doesn't match with the realtime nature of the database. To read document N+1, you'll need to also read all N documents before it.
The server-side SDKs for Firestore do have the necessary offset API, but that internally does precisely what I described already: it reads all N+1 documents, and returns only the last one of that sequence.
If you need a ranking system, you'll indeed want to store the ranking inside each document. Have a look at this explanation from Dan on how to build such a system on Firestore: Leaderboard ranking with Firebase
If you want to return a random document and use your random numbers as the basis for that, that is possible. Take a look at (again) Dan's explanation about how to do that here: Firestore: How to get random documents in a collection