So, publishers know full well that it’s not just teenage boys reading shounen stories. There are actually more girls reading shounen than there are girls reading shoujo. For this reason a lot of stories that appeal to both end up being published as shounen, whereas stories that are more likely to only appeal to girls get published as shoujo. The same thing happens with seinen and josei; lots of adult women read seinen stuff, whereas it’s less common for adult men to read josei stuff. Seinen magazines have also been around for a lot longer than josei magazines, so sometimes if a mangaka wanted to publish something spicy in a shoujo magazine it would end up getting rejected and getting published in a seinen magazine instead if the publisher and the mangaka thought they could swing it.

This one I don’t have stats on, but I suspect there are more adults reading shounen/shoujo than there are kids/teens reading seinen/josei. The big shounen magazines are really big, so if the mangaka for a story like Frieren thinks their story appeals to everyone it makes sense to pitch it with the big shounen magazines first.