What is the reason for the appearance of virus files and folders?

Question

WordPress

Good time.
I went via ftp to the client's project, and there some "left" folders and files were conveniently arranged like that. I cleaned it once by hand, after a while the picture repeated itself.
I applied basic security measures even before the problem arose, but, for some reason, this was not enough.
Tell me, what kind of attack is this? How to solve it completely?
Maybe it's the responsibility of the hosting?

ps Collected such .htaccess from different sources. Maybe something is wrong there? Should I change or add something?

Answer 1

The whole project under Git and the problems will disappear by themselves. The reason is that there are many entry points on WordPress sites and it follows that any php file can be bookmarked with a file manager for example.

Answer 2

Install Wordfence, run a check in it - clean it according to all its recommendations. And leave it activated for a while.