Hey
it is not completely separate
(أ,د,ذ,ر,ز,و)
they only join with the letter before
E.g.
قديم - old
دجاج - Chicken
نوم - sleep
you can see the letter د connected with the letter before
those letters can be separate in one case
only if they follow each other.
Like:
حروف - Letters ,ABC
مدرسة - school