There are a few different ways. One is to simply register a domain that’s memorable or brandable and sell it for a profit.
This is called domain investing aka domaining.
You can also “park” a domain, — with services like Cash Parking—which means placing ads on your domain and earning a percentage of revenue from the sales of visits your site generates. There are companies you can work with to do all the hard work, from finding advertisers to making sure you get paid.
Some people also lease their domains, use it to generate leads for businesses and more.