There’s a lot you can do with HTML! I, myself, really only know the basics but it’s possible to add sections! Here’s the code:
<style>
<tr><td>
<p>Section Title Here</p>
</td><td>
<p>Whatever you want here</p>
</td></tr>
</style>
Bolded portions can be changed to whatever you like (I put the <p> tag as bolded since you don’t necessarily need them if you’re not writing a paragraph in the section. For example, I use bullets in my new sections so I don’t use the <p> tag, if that makes sense). For example, here’s the code for if you wanna make a section focused only on games:
<style>
<tr><td>
<p>Video Games</p>
</td><td>
<p>I love playing Minecraft and Pokemon!</p>
</td></tr>
</style>
You can put this code in any of the sections and it’ll work, but where the new section ends up depends on where you put the code. If you put the code in the General section, the new section will be under General. If you put it in Heroes, it’ll be under Heroes.
A bit unrelated to your main question but you also mentioned people who hide their sections so I’ll add the code for that, too, in case you didn’t know:
<style>
table tr:nth-of-type(Insert number here) { display: none !important;}
</style>
You see, each Interests section is labeled a number from 1-6 (General = 1, Music = 2, Movies = 3, etc.). If you wanted to get rid of Movies, you’d put 3 inside the parenthesis. On the topic of adding sections, however, any added sections will jumble up the numbers. You can just use trial-and-error with all numbers to eventually hide the section you want OR you can preview your profile normally and count down from each section (Notice how General = 1? Well, any section below General will be 2 so if your new section is under General, that means that section is now 2). The numbers can be more than 6, if you added more sections
I’m not a great teacher :P but I hope this helped in some way!