What the export holds
X has no export button for Lists, so people copy members by hand or install an extension that reads their logged-in session. This tool reads the List from the outside, through the same public data a logged-out visitor can see, and writes one row per member. The user id column is the one to keep: handles change, ids never do, and the ID finder turns an id back into the current handle later.
The first 50 members are shown on the page; the CSV holds every member read. For Lists with more than 300 members, or to export on a schedule, the List members export guide has a Python script that follows the cursor to the end, and the Twitter List API guide covers a List's posts and details too.
Good uses for a List export
Lists are curated by their owners, which makes them a cleaner source than search: a “climate scientists” List made by a journalist, a conference's speaker List, or a company's team List. Export it, sort by followers, and you have a shortlist. To find Lists on a topic, search X for the topic and open the Lists tab; to find people by what their bio says instead, use the bio search.