how is blockchain data stored and accessed? Can blockchain data be stored in a relational database? If so what would the schema be? I know blockchain itself is a data structure, but how is the data stored? In JSON files? In a RDBMS? in CSV file? What do you see in the wild? I'm looking to see if I can implement something for a case study in a RDBMS which we are required to make use of.
Is there any example of a schema for a given blockchain with tables I can find? (I started creating my own and I think it's reasonable).
Blockchain: Chain of blocks
+++++----------++++---------++++++---------+++++++
Who creates the blocks? Miners
Who validates, saves and relay these blocks? Full nodes
https://bitcoin.stackexchange.com/a/103172/

Different full node implementations for Bitcoin: Core, btcd, bcoin, gocoin, libbitcoin, knots, etc.
Database used by Bitcoin Core: Bitcoin Core stores the transaction-journal in groups of consecutive blocks in a series of large files whose format I believe closely matches the form used in the Bitcoin network protocols. But there is no prohibition on other software products doing something quite different.
https://bitcoin.stackexchange.com/a/98830/
Berkeley DB and SQLite are used for wallets