Yep, @Kelheor is right, just premium and just comics for now under mature. I'm hoping to see Mature come to novels soon too!
Re: applying to be under the Mature Section, the FAQ currently says this: "Currently, the Mature section is not open for submissions, but we’ll be making announcements about any publishing opportunities in the future, so stay tuned!"
Keep in mind that it would impact getting shown in the app due to various app store regulations as well.
Thanks for the tag @Enigma_the_Slimy_Boi