Yeah, I always get confused with these things. I just accepted "shoujo" and shounen" mainly as the type of magazine is published (then after I try to look if there's any "checkbox" to mark)
There are a lot of people who will say that shoujo or shounen are purely the demographic, so if it appears in a shounen magazine then by definition it is shounen. And
it's certainly the #1 starting point, like you said. But I think that's a little reductionist, because do you really want to consider Jujutsu Kaisen and Tonikaku Kawaii to be effectively the same thing? I seriously think TK would do just fine in a shojo magazine, maybe with an art change - art is probably the #2 thing, you can very often tell whether it's aimed at male or female, then kids, teens, or adults just from the art.
This manga actually avoids that, I'd guess shojou or shounen just from the art but which of those would be ?
If I really want to go 'no really, what kind of manga is this?' like the OP asked, or if you don't know what mag it's in or if it's not in a mag, then I think of the most cliche examples of the genre and then figure out where the manga is in the venn diagram. Lots of fitefitefite or titties a poppin' harem? It's shounen. Lots of work drama, adultery, working adult ennui? It's josei or dansei. Lots of swooning and agonizing about trivial stuff or reverse harem? It's shoujo. Lots of poop jokes? Probably young shounen. Every 'demographic' has tons of checkboxes, some overlapping. I don't have an explicit list of checkboxes (maybe I should!) but my gut is that manga is about 70% shoujo. I do think things have been generally drifting to the center, but then you get something like Kagurabachi (pure shounen) which is a breakout hit, maybe because readers are longing for some simplicity.
Anyhow, how did I type so much on this? I just find it fascinating because even the Japanese on the forums I read don't know, they're too deep in it. Please forgive my rambling!