China DOS Union

How to copy all files in directory A

to all first-level subdirectories of directory B?

I'm a newbie too, haha... if I said something wrong, please don't mind. This is a question raised a long time ago, hehe, just giving the OP a bump:
for /l %i in ('dir/w/b B') do (copy %i a)

This is one approach; you'll need to change the specific paths for folders a and b yourself
@echo off
for /f "delims=" %%a in ('dir /ad /b b') do (
for /f "delims=" %%i in ('dir /a-d /b a') do (
copy "a\%%i" "b\%%a"
)
)
pause

To all first-level subdirectories of directory B? Did I get that wrong or what?

Use xcopy