As you might have expected, the answer for most of your questions is "it depends". The Eastern Front was absolutely massive and the conditions could vary dramatically depending upon the location, time, and situation.

I know from my grandfather, who was a German soldier that they were often brought to the front in trucks and marched to their positions. What did these positions look like?

Actually, being trucked to the front would be rather unusual. Something like 80% of the Germany Army was horse drawn (and for the most part those horses were tasked with transporting equipment and supplies, not personnel). The belief that the German military was highly mechanized is a common but false impression. Obviously this is not to say that soldiers would never be given a ride; it simply was not the norm.

What did these positions look like?

Depends upon the situation. Is the unit going on the offensive, or are they relieving a unit that was on the defensive? It could range anywhere from absolutely no prepared positions (aside from those occupied by the enemy) all the way to extremely formidable constructions.

I'm particularily interested in the German defensive positions after the momentum of the war shifted and the long retreat began. Did they mainly dig foxholes or did they have trenches like in WW1?

Soldiers (particularly experienced ones) would have devoted a large amount of their time to improving whatever terrain they were forced to work with. At the bare minimum an experienced soldier would dig or construct a defensive fighting position. The amount of improvements that could be made to these constructions was limited only by the imagination and what materials were available. During the notoriously brutal winters, soldiers would sometimes find the ground too frozen to dig, and would either have to blast fighting holes using explosives, use heavy construction equipment, or simply construct on top of the ground instead of digging into it.

Especially in the second half of the War the Germans favored what was known as a hedgehog defence. These were basically a series of interlocking defensive positions that could mutually support each other, and had lower manpower requirements than a full-blown trench system would have needed.

Also, how deep was the frontline? Just one trench or many behind each other? Where was the artillerey located? How far were they away from the Sowjets? Did the Sowjets build their own positions so that a WW1-like situation ensued or did they not even bother since they were so much stronger during the end of the war?

Again, it depends. Generally speaking it was found the best way to counter blitzkrieg-style attacks was through "defense-in-depth", which meant instead of having all your units packed into the front lines, you spread them out. The intent was to force any attacking units to have to fight every step of the way, as soon as one defensive line was breached they would immediately encounter another, and another behind that. Additionally German tactical doctrine was very insistent upon counterattacks. Ideally, as soon as the enemy captured a position, they were supposed to be counterattacked (psychologically this was when an attacking unit would be most vulnerable; the Adrenalin rush of a successful attack would be wearing off at that time). (NOTE: Would appreciate it if someone could confirm or deny if counterattacks were as practiced on the Eastern Front as they were on the Western Front. I've read about it time and again from the American/British perspective, however cannot state with certainty that it was true in the East and don't want to state it was the case without confirmation).

Artillery that was being employed in an indirect (i.e. fired up into the air to have it come arcing down on the enemy) would have been (ideally) placed behind the front lines. Artillery being used in a direct (i.e. you can see the enemy and you are firing directly at them) role would have been placed on or near the front lines, and would be integrated into the defensive plan.

How far away the enemy was could vary dramatically. In some cases it was miles apart, in others they were practically on top of each other. There are plenty of stories from the Stalingrad siege in which a building might have Soviet soldiers on the top floor, German soldiers below them, and more Soviet soldiers below them ("layered like a cake" is how I recall it being described in one book). They often could hear each other through the floors/walls.

I know from my grandfather that the Sowjets often came out of the forest after a long period of shelling and stormed towards their positions. Were these attacks parts of bigger attacks or was it basically a few hunderd people trying to storm a certain part of the German positions?

Again, it varies, and both could be the case. An attack could be a small, local affair consisting of just a handful of soldiers, or it could be an absolutely gargantuan undertaking consisting of millions of soldiers.

If you have further questions or want to know more about something, please don't hesitate to ask.