An operator is the business that runs a gambling site: it holds your account and money, takes your stakes, pays your winnings and is responsible for safer gambling and verification. In Great Britain it must hold a Gambling Commission operating licence.
Operators do not usually make the slots they offer. Games come from game providers, often delivered through an aggregator. The operator chooses which games to list and, for many titles, which RTP version to run (see RTP range).
Some brands are white labels, run on another licensee's platform. Either way, the licence holder is accountable to the regulator, and its name and account number should be shown in the site footer. You can check them on the Commission's public register.