About the German prefix ur- a lot has been written. But what I read is not satisfying.
First of all ur- has different sources.
Urlaub - this ur- is connected with er- as in Erlaubnis/erlauben. Urlaub was a military term and it was the permission to leave the military service for some time.
Urzeit - in most cases ur- has the meaning of "earliest/first". I think this ur- is a variant or vor- with drop of v and change of o to u. I would connect it with "vorderster"
(English first and German vorderst-er/erst-er are related).
German vor is related with Latin pro and its variants pre-/pri-/por-and Latin pri-mus and Greek pro:tos meaning the first.
Ultimately I think the idea of ur- is "the foremost".
I think this is the place where water springs up from the ground to form a rivulet. And I think this ur- is connected with "empor", a variant of "vor", meaning "hervor" out of.