When you are filling out the web form with your password it’s stored plain text in the web browser and accessible via JavaScript. At that point, a JavaScript function checks the requirements like length and then does the salting/hashing/etc and sends the result to the server.
You could probably come up with a convoluted scheme to check requirements server side, but it would weaken the strength of the hash so I doubt anyone does it this way. The down side of the client side checking is that a tenacious user could bypass the password requirements by modifying the JavaScript. But they could also just choose a dumb password within the requirements so it doesn’t matter much… “h4xor!h4xor!h4xor!” Fits most password requirements I have seen but is probably tried pretty quickly by password crackers.