Q:

Emacs.  Helm package.  How find files in folder (not a git folder) and all subfolders? 

Windows 10 (64 bit), Emacs 25.1, Helm package. 
To find file (e.g. myfile.txt) in any folder (not git, not project file) I use command helm-find-file.  It's work.  OK. 
But now I need to find myfile.txt in current folder AND all subfolders.  How I can do this by helm package? 

A:

There's two ways that should work for you.  In both cases, first use helm-find-files.  
1) Once you're in the directory that you want to start your search from, hit C-u C-x C-d (this runs helm-browse-project recursively).  Now you can type the filename that you're looking for and it should search in all subdirectories. 
2) Alternatively, again from helm-find-files, hit C-c / (this effectively runs find (not sure how well this will work in Windows)).