When you install WordPress, it ships with its own theme.
This is WordPress’ own theme and they update it each year. The theme will just have a simple name like ‘Twenty Twenty One Theme’.
Whilst this theme is great, it’s not often used to customise sites. It can create very nice blogging sites or can be a great start for anyone who would just like a presence on the internet.
However, many people choose to upgrade with either an off-the-shelf or bespoke theme so they have much more customizable options and the ability to make the site their own.