For example, whenever someone tries to go to
apple.com, I want them to go to another website, but just when someone connects to my Wi-Fi. Maybe by using dnsmasq? Please don't be afraid to ask for clarification.
Those things are simple to do for someone who understands basic networking.
How and what you can do (and how reliably you can do it) will depend on what equipment you have available, not all home wi-fi routers are created equal in that regard. Some can be given custom firmware like Tomato or DD-WRT which give them extra features. The ability to redirect (or restrict) sites would be one such feature.
You can buy a Wi-Fi router that offers those features (or can be 'flashed') but you'd still need to configure it and this sounds like where you'll come unstuck. So, the best advice is to ask someone who knows what they're doing to set it up for you. Most techies are up for a little mischief on a home network in return for a beer or two.
One of the easiest (and easiest to bypass) methods to do this is to set up a dns server that gives alternate address to the censored website that point at a specially configured webserver that redirects the traffic.
another method is a layer 3 switch and a transparent proxy.