How to deploy hardhat contract to mainnet without ALCHEMY or INFURA ?
Alchemy, Infura, and others, are services offering connection to nodes of the Ethereum network.
Contract deployment means that you broadcast a transaction with specific parameters (data field containing the compiled bytecode, to field empty). But you can only broadcast a transaction to the rest of the network from a node connected to the network.
So apart from using a third-party service, you can also run your own node. For example using the go-ethereum client software.
Then you'll be able to broadcast the transaction through this your own node, instead of a third-party one.