How does this batch password generation work? - batch-file

Question

I am creating a password generator in batch, and had to use some help from the internet to finish it, but I am having difficulty interpreting the section of the code I had to borrow. Would someone please explain it?
:generate
#Echo Off
color 0a
set /P usernumberlength="What length do you want your password to be? "
pause
cls
Setlocal EnableDelayedExpansion
Set RNDLength=%usernumberlength%
/// from here
Set Alphanumeric=ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789
Set Str=%Alphanumeric%987654321
:loop
IF NOT "%Str:~18%"=="" SET Str=%Str:~9%& SET /A Length+=9& GOTO :loop
SET tmp=%Str:~9,1%
SET /A Length=Length+tmp
Set count=0
SET RndAlphaNum=
:loop2
Set /a count+=1
SET RND=%Random%
Set /A RND=RND%%%Length%
SET RndAlphaNum=!RndAlphaNum!!Alphanumeric:~%RND%,1!
If !count! lss %RNDLength% goto loop2
/// to here
Echo Password is: is !RndAlphaNum!
Echo Now choose what you want to do.
Echo 1) Go back to the beginning
Echo 2) Exit
set input=
set /p input= Choice:
if %input%==1 goto generate
if %input&==2 exit
pause

Answer 1

This working example (from the code you provided) contains the all-you-need section.
#echo Off
color 0a
setlocal enabledelayedexpansion
:generate
set /p usernumberlength="What length do you want your password to be? "
rem ----- begin section all you need -----
Set Alphanumeric=ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789
rem "Length" is the string length of %alphanumeric%
Set Length=62
set count=0
rem "RndAlphaNum" is the output
SET RndAlphaNum=
:loop
Set /a count+=1
Set /A RND=%Random% %% %Length%
SET RndAlphaNum=!RndAlphaNum!!Alphanumeric:~%RND%,1!
if !count! lss %usernumberlength% goto loop
rem ----- end section all you need -----
echo Password is: !RndAlphaNum!
goto generate