As CES 2021 is drawing near, leaks of up and coming phones will also appear. In these leaked pictures, it shows a phone that is suspected to be the Samsung Galaxy S21 Ultra with an S Pen (accessory).

There are already rumors that Samsung might be combining the Samsung Galaxy Note and Galaxy S series. It does make sense as the form factor is getting similar except that the Galaxy Note has an S Pen while the Galaxy S Series does not.
Now, with this leak, it might have proven this rumor.

Again, all rumors are rumors until they have been confirmed by official source.
From these pictures, the S Pen seems to be a separate accessory and it is not inserted into the phone. Instead, the casing actually holds the S Pen (based on the picture).
Check out this picture below that show the casing of the leaked Galaxy S21 Ultra.

Do you want a stylus for your phone?
BTW, the official announcement might come as early as 14th January 2021.
(source)