Split string by space first, then split the 6th string by colon.

nested_string_splitting.bat

@ECHO off
for /f "tokens=1,2,3,4,5,6" %%a in ('adb devices -l') do (
  if "%%b" == "device" ( 
    ECHO Serial Number : %%a
    for /f "tokens=2 delims=:" %%A in ( "%%f" ) do (
      ECHO Transport Id : %%A
      start adb_by_transportid.bat %%A
    )
  )
)
PAUSE