You can access additional post options, such as Categories, Featured Image, and more by selecting options at the top right corner of the Post screen by clicking the ellipse menu (…)
and then Post settings.
Click on Post settings to see the settings for your post. Inside the Post settings, you can set tags and categories, add a featured image, set a publish date, and more.
To view the post settings when you are editing a post, tap on the three dots in the top right and select Post Settings to view the settings for that post.