those are BL manga magazines that release new chapters of ongoing manga from artists they contract with. in shounen, there's a popular one called shounen jump which releases the famous shounen manga (like naruto or boku no hero academia)
a famous shoujo counterpart of the magazine is hana to yume which released skip beat, gakuen alice, special a, oresama teacher etc
an old famous BL magazine is BE X BOY comics, you might see old mangas here on mangago with that logo
basically it's where scanlators use for illegal updates. someone in japan sells these "RAWS" to scan groups and they translate it.
oooh i see, thank youu for explaining!
What's Hertz and Craft?