These kind of messages are about joint publishing at best.
There are three ways to publish: the traditional one with a publishing house, self-publishing and joint publishing. NEVER choose the last one.
The first one consists basically in you selling the exploitation rights to your writing to the publishing house, which will pay you a percentage of what they get from your work. You just write, send them the finished work, and they do all the rest, from formatting to selling and advertising. You get less money, but also less risk, because the risk is on the house.
Self-publishing is you doing literally everything. You write, edit, lay out, go to the printing house, all the legal stuff... Everything. You pay for everything, assume all the risks, and also have 100% of the creative control, which is the main advantage of this way. You also get all the money from selling instead of just a percentage.
Joint publishing? Sketchy as all hell. Usually it's a printing house posing as a publishing house, but they make you pay for the printing. They can be direct and give you a budget, or they might be shady and say stuff like "you have to buy the first X copies" or "you have to personally sell X copies" or something like that.
Which means that you, the author, are covering all the expenses of the printing and distribution, and assuming all the risks, while they, as the "publishing house", get the bigger percentage of the selling and the exploitation rights.
It's a scam. As much as you want to publish your writing, don't trust anybody who comes to you like this. They don't care about the quality or whatever, they probably didn't even read your fics, they just want to publish stuff basically free (because the author pays for the production) so they get most of the money from selling.
Please be careful with this kind of messages!
A Twitter thread in spanish that's better explained