Please don’t create multiple topics for the same thing, those get closed or deleted when found.
Multisite lets you run more than one WordPress blog or site but with a single software installation. That makes maintaining your multiple sites easier.
BUT it is a little more complicated and you need to be aware of how DNS works. Also it is not designed to permit content to be shared between sites in the network and out of the box you can’t use different domains. Using different domains requires another plugin and even more DNS setup.
Start by giving this a read.
https://codex.www.ads-software.com/Create_A_Network
That should give you an idea of how to begin to set that up.