How do you learn about domains?

The best way is to get a job at a company that aligns with your interests. Spend enough time there to get a good grasp on the customer, their needs, and how the company solves their needs.

1. Talk to every single person you meet and ask lots of questions. Your taxi driver. The homeless person on the corner. The girl in the red dress. The drunk dude outside a bar. The businessman. An old woman sitting by herself on a park bench.
2. Work for a company that deals with lots of people from many walks of life. An ad agency. A wealth manager. Customer support for something. etc.

The best but the longest way is to dive into the domain like if you are interested in software development, become a software developer (I told you it's the longest way:)
But there are maybe less time-consuming ways:

• get a job that can be applied to the domain too. Say, if you are a software developer but want to know about aerospace, find a job as SD in the aerospace domain
• read books and watch the documentaries
• subscribe to their newsletter, read their websites, and subreddits
• make friends who are related to the domain somehow
• visit their events, listen to their podcasts, use their products, read their documentation.

Basically by talking to people who are in a group that you want to solve problems for, and/or observing them as they go about their work.

Once you talk to enough people in a group, patterns start to emerge.

Alternatively, you can take a broader approach by going to the online places that these people communicate with each other (Reddit groups, forums, Quora, blogs, etc) - their “watering holes” - and analyze their problems there. This is an approach Amy Hoy calls “Sales Safari.”