>>136049
There are official manga reader sites, but usually only the first and the latest chapters are public in most series. Shounen Jump, GanGan, Mangabox, Pixiv Comic, probably many others I haven't seen yet.
Also, enormous amount of amateur manga series in the sea of Pixiv. Mostly fanart, and some original series. A few are picked up by a publisher, I think Eiga suki Ponpo-san started that way.
>>136157
I've seen a few monthly magazines uploaded in a few places, but usually those places have mostly the books from manga series and novels. I don't know why they do it that way.