Most miners don't actually use getblocktemplate anymore and they have their own software written to accommodate for their needs (ASICBoost, custom msg in coinbase etc).

The documentation is here: https://developer.bitcoin.org/reference/rpc/getblocktemplate.html. The code is written here: https://github.com/bitcoin/bitcoin/blob/793e0ff22cbace2a0fbe1e4a2e88a7bc6bf44502/src/rpc/mining.cpp#L488.