How are smart contracts tested before deployment to the Eth Mainnet?

Hi All, sorry if this is a stupid question. Say I was developing a smart contract and wanted to test how it functioned on a working blockchain without deploying it and paying gas fees/making it public.

Are developers able to run a "private" version of an Ethereum blockchain on their own systems with miners (and trivial mining difficulty for quick testing). Or simulated POS of course.

Really interested in learning more about how things work practically from a software development point of view. Thanks!!