ffmpeg combining videos generalizing this https://ffmpeg.org/ffmpeg-filters.html#toc-Examples-138

ffmpeg_xstack_layout.py

import os

def command(filename,x=2,y=2):

  command="ffmpeg"

  for i in range(x*y):
    command+=f" -i '{filename}'"
  command+=' -filter_complex "'


  for i in range(x*y):
    command+=f"[{i}]".format(i)
  command+=f"xstack=inputs={x*y}:"
  
  #### generalizing this part "layout=0_0|0_h0|w0_0|w0_h0"
  def a(grid,num,s):
    if num==0:
      return "0"
    res=""
    for i in range(num):
      res+=f"{s}{i}+"
    return res[0:-1]
      
  add1=""
  for h in range(x):
    for w in range(y):
      add1+=a(x,h,'w')
      add1+="_"
      add1+=a(y,w,'h')
      add1+="|"
  ###################################################
        
  #scale to width 1280 and auto height
  #command+=f'layout={add1}[v];[v]scale=1280:-1[g]" -map "[g]" -map "0:a" o.{filename.split(".")[-1]}'
  command+=f'layout={add1}[v];" -map "[v]" -map "0:a" o.{filename.split(".")[-1]}'

  print(command)
  return command
  
os.system(command(filename='output.webm',x=3,y=3))