What redeem script is used in bitcoin core wallet when generate the address?
You can use the getaddressinfo RPC command in the latest version of bitcoin core to get the hex encoded script for that address. But note that because you were able to get a private key, I assume this is a P2SH wrapped segwit address, which means the script is just an OP_0 opcode followed by the hash160 of the public key corresponding to that private key.