(moving this over here from the Unpopular Anime Opinions thread bc I really just wanted to ramble on about manga magazine audiences a little more)
Seinen really is like the miscellaneous tag. Really, demographic labels aren't genres, they're just target audience indicators even if works tend to have a lot in common bc target audience, but seinen especially is very much like "it's aimed at adults, I guess?".
Right now in Kodansha's Morning Two magazine, which is definitely a seinen mag, there's Witch Hat Atelier (pretty safely all-audiences fantasy story), Heaven's Design Team (fun educational story), something called Anata wa Bun-chan no Koi which appears to feature a love triangle with a ghost, Amane Gymnasium which has some very creepy-looking dolls that do not make me want to investigate further, Nora to Zasshou which is about a police investigator who encounters a runaway teenage girl, and the Cells at Work spinoff about women's bodies (probably also fun and educational). There is no common thread beyond running in the same magazine, not themes, not tone, not art styles.
(Plus sometimes titles aimed at women get moved to seinen magazines because the publishers think they'll appeal to a broader audience - 10 Dance is 100% without a doubt a bl and started out in a bl mag, but moved publishers and now runs in a seinen mag without changing anything about it.)
Other seinen magazines do cater more obviously towards a certain audience of young men - e.g., there's gravure models on the cover - and probably what they publish has more shared aspects than the broader-reaching mags, but yeah, trying to figure out what ties seinen together beyond magazine is definitely headache-inducing. Even trying to work out josei beyond "is in a josei mag" is complicated, though it's at least more likely to have romance and female-centric stories.
I appreciate that AP goes by the magazine when applying demographic labels because it does get complicated really quick and going by feel can lead to a lot of arguments.