If I must read specific block for example 200000, in which file and which position in this file this block begin?
I must read indices, where described is indices format?
In
https://en.bitcoin.it/wiki/Block I can't see block heights
I can't see block hash, only previous hash.
In blockchain are also blocks from alternative branch?
The indices for the blocks are in a separate LevelDB database. The blk*.dat files are simply where the blocks are stored, but it means nothing to the software without the database which indexes the blocks. Those indices are kept in the index folder inside of the blocks folder. Bitcoin Core will write to the disk every valid block it receives, so this does include forks and stale blocks in case of blockchain reorgs.