Where can one find the best explanation of the blockchain? Sorry if these questions seem stupid but after a lot of research I still feel rather lost and confused !!
I understand what is bitcoin and have a fairly good understanding of the basic blockchain idea but I have a hard time understanding some concepts. 
For example where can one download the source code for the blockchain ? How did Ethereum get the source code for the blockchain ?
How can an update be "scheduled" for a specific block and not be implemented immediately ?
Or how can one code apps for a blockchain platform like ethereum ? 

I guess all these fall in the question, where can I find a thorough, complete explanation of the blockchain?

Thanks !
Since nobody answered and I continued my research I'll post something I found online that helped me understand the blockchain. 

http://blockchaindemo.io - This website visually simulates a simple blockchain to explain how it works. Pretty well made in my opinion based on comparison with other explanations I found online.

Otherwise here is the white paper from Bitcoin's creator, Satoshi Nakamoto, even if outdated, it can help understand some of the original concepts:
https://bitcoin.org/bitcoin.pdf