How to index data when downloading?

Question

C++ / C#

Hello.
I want to make software, but I don't know how to index information within the system.
Generally. There are a bunch of files like "md5(hash):Text"
There is not a limited number of hashes in a file, as well as files.
The bottom line is to write software with which you can download hashes from other users that I do not have.
-Something like bittorrent
-SQL databases are rejected
-It would be great to stick with the idea of ​​a bunch of .txt files like -hash:text

How the bittorrent client does it, how it understands which part of the file has already been downloaded and which has not.
I know that he somehow divides the files into parts and signs.

Maybe come up with short electronic signatures for each line .. (Thoughts out loud)