In the documentation on https://tezos.gitlab.io you will find:
https://tezos.gitlab.io/introduction/howtouse.html: a description of what Tezos does at a high level under the form of an introductory tutorial targetted at new Tezos users.
In https://tezos.gitlab.io/whitedoc/the_big_picture.html: the organisation of the code base.
In the rest of the "...
You actually answered the question in the comments, but for those who don't read comments:
There are two limitations:
max_operation_data_length: 16384 bytes
hard_storage_limit_per_operation: 60000 bytes
This means that you can't send more than ~16KB per transaction, so the only way is to split the "really long string" into chunks and store those ...