Shojo and Josei manga are designed for female-audiences. They can consist of male characters (DNAngel) or female characters (Princess Jellyfish).
Shonen and Seinen manga are designed for male-audiences. They can consist of male characters (Naruto), or female characters (Madoka Magica).
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Madoka Magica in particular is a Seinen-oriented story pretending to be a Shojo story. So the categories can get complicated sometimes. All categories know and understand each other's demographics, as well as the tropes and overarching storylines that enter one demographic vs another.
But that's what makes "cross-demographic" shows (like Inuyasha, Kenshin or Madoka) so much fun. They take elements of one genre, and shove it into a totally different genre.
Inuyasha is nominally Shojo (aimed for a female audience), with Kagome (female protagonist) meeting lots of cute fantasy male characters (Inuyasha in particular). But it takes a battle-style reminiscent of Shonen, and a lot of male-audience members identified with Inuyasha.
Kenshin is nominally Shonen (aimed for a male audience), with Kenshin (male protagonist) largely doing battles. But Kaoru's arc largely plays out like a Shojo romance. As such, a lot of female audience members identified with Kaoru.
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The gender of the main character though isn't really that important though. In the West, it seems like "male-audience must have a male-main character", but that sort of thing just doesn't seem to be an issue in Manga. Its kinda-sorta there in some cases, but there's enough manga / anime that break the rule that it really isn't a rule at all.
A lot of Shonen are abridging the training arcs (ex: Demon Slayer, Mob Psycho skipped the training arcs).
It seems to be some new hypermodern kind of storytelling. Turns out that the "Training arc" really isn't needed. But even something as old as "Full Metal Alchemist" never really had a major training arc.
But Demon Slayer is popular because it was written by a woman and drawn kind of shoujo-y.
(Jump, which has no women working for it and never has, doesn't actually realize this. They think it's popular because it's got demons and stuff, so they approved a bunch of mystical setting comics to follow it up… like Ayakashi Triangle… clearly women are going to love that one.)
Hiromu Arakawa (Full Metal Alchemist) is a woman. EDIT: Oh, wow, that's not a "Jump" manga. Huh, well TIL... so your point stands.
CLAMP also made a scene back in the 90s / 00s by emphasizing the shear number of women in their circle. They made all sorts of manga too across all genres... even dabbling into erotica. Cardcaptor Sakura, Chobits, xxxHolic, Rayearth, the character designs for Code Geass, etc. etc.
IIRC, Cowboy Bebop's script was also written by a woman. Etc. etc. I do hear that Japan is pretty bad when it comes to gender issues but I'm really not super into the culture believe it or not (I do like Anime/Manga, but I understand that's just a subset of the culture).
Totally random, but I literally just read the scene in When supernatural battles became commonplace where they have this conversation almost word for word...
Western cartoons always have boys as protagonists because the execs thinks nobody likes girls. They just have the opposite opinion is all. Mainly the character ages are because they're targeted to readers of the same age, but there's definitely impure reasons for it too.
All these covers have pretty different artstyles, it feels kind of random and the art isn't great either. Not sure if it's meant to be a parodying popular series or what.
Mostly they look like boys' comics, "Statistics for Nurses" feels like website clip art, and "Surveying" looks like an older girls' comic or book cover for a Western novel.
Shonen and Seinen manga are designed for male-audiences. They can consist of male characters (Naruto), or female characters (Madoka Magica).
-------
Madoka Magica in particular is a Seinen-oriented story pretending to be a Shojo story. So the categories can get complicated sometimes. All categories know and understand each other's demographics, as well as the tropes and overarching storylines that enter one demographic vs another.
But that's what makes "cross-demographic" shows (like Inuyasha, Kenshin or Madoka) so much fun. They take elements of one genre, and shove it into a totally different genre.
Inuyasha is nominally Shojo (aimed for a female audience), with Kagome (female protagonist) meeting lots of cute fantasy male characters (Inuyasha in particular). But it takes a battle-style reminiscent of Shonen, and a lot of male-audience members identified with Inuyasha.
Kenshin is nominally Shonen (aimed for a male audience), with Kenshin (male protagonist) largely doing battles. But Kaoru's arc largely plays out like a Shojo romance. As such, a lot of female audience members identified with Kaoru.
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The gender of the main character though isn't really that important though. In the West, it seems like "male-audience must have a male-main character", but that sort of thing just doesn't seem to be an issue in Manga. Its kinda-sorta there in some cases, but there's enough manga / anime that break the rule that it really isn't a rule at all.