What demo mode actually is
A demo version runs the identical certified game build with a fake balance. The RNG, paytable and RTP are the same as the real-money version in a properly licensed environment — the only thing that changes is that wins and losses cannot be cashed out. This makes it a legitimate way to check a game's volatility, bonus frequency and speed before risking anything.
Studios publish demos on their own sites and on aggregator portals, so you do not need an account to try most titles. If a site only offers demo play after registration and a deposit, that is a marketing funnel rather than a learning tool.