Do I know which card an opponent revealed?
You only know a revealed card for the duration that it remains revealed.

The example of Shock moving from the graveyard to the hand is a bit nuanced (since the card isn't technically revealed, mechanically), so I'm going to start with a more direct representation of the question and move from there. Let's say you cast a card like Thoughtseize which causes your opponent to reveal their hand. In MTG Arena, this is represented by the game revealing all of those cards to you from that point onward. However, that's not quite feasible in paper Magic - in the vast majority of cases, you'll pick a card for your opponent to discard, and then they'll go back to holding their hand towards themselves. That is, even though the information was revealed to you, by default you don't continue to be able to see those cards. It would be impractical for the opponent to hold newly drawn cards in their hand and simultaneously have those previously revealed cards lying on the table, for example. The information is known, but it's up to you to remember what that information was.

When I was playing Standard regularly (circa Theros) and my opponents cast Thoughtseize targeting me, I would show my opponent my hand, and they would use a notepad to write down the cards that they saw. Later, as I played those cards, they would scratch off the name of the card that I played if it was in that list. This, I believe, is what the functionality of MTG Arena seeks to emulate. Knowing that information was revealed to you, but misremembering it, feels bad. Arena can leverage its nature as a computer game to assist you with that, by making sure that you don't forget what cards were revealed. (There are other aspects it helps you with, too -- in real life, the cards you can cast don't glow.)

Rule-wise, of particular interest are the rules for revealing cards:


  701.15a To reveal a card, show that card to all players for a brief time. If an effect causes a card to be revealed, it remains revealed
  for as long as necessary to complete the parts of the effect that card
  is relevant to. If the cost to cast a spell or
  activate an ability includes revealing a card, the card remains
  revealed from the time the spell or ability is announced until the
  time it leaves the stack. If revealing a card causes a
  triggered ability to trigger, the card remains revealed until that
  triggered ability leaves the stack. If that ability isn’t put onto the
  stack the next time a player would receive priority, the card ceases
  to be revealed.
  
  701.15b Revealing a card doesn’t cause it to leave the zone it’s in.
  
  701.15c If cards in a player’s library are shuffled or otherwise reordered, any revealed cards that are reordered stop being revealed
  and become new objects.


The phrasing of 701.15a makes it pretty clear that a revealed card is only revealed for a short amount of time, and then ceases to be a revealed card. You know what the card is because it was revealed, and can do whatever you want with that information (such as write it down), but it doesn't stay revealed after that. By extension, cards moving from a public zone to a hidden zone (rule 400.2) work very much the same way. Everyone is allowed to know what that card is as it moves into the zone, but after that, it becomes hidden.

Some relevant excerpts from Rule 400 regarding public and hidden zones:


  400.2 Public zones are zones in which all players can see the cards’ faces, except for those cards that some rule or effect specifically
  allow to be face down. Graveyard, battlefield, stack, exile, ante, and
  command are public zones. Hidden zones are zones in which not all
  players can be expected to see the cards’ faces. Library and hand are
  hidden zones, even if all the cards in one such zone happen to be
  revealed.
  
  [...]
  
  400.7 An object that moves from one zone to another becomes a new object with no memory of, or relation to, its previous existence.


To use your example, let's say your opponent uses Call to Mind to return Shock from their graveyard to their hand. During the announcement of the spell (rule 601.2c), they would choose the target for it - Shock. Because the card is targeted, everyone inside and outside the game knows which spell is going back to their hand. As soon as Call to Mind resolves, the Shock is moved to their hand, where it becomes hidden along with all the other cards they hold. You know what card it is, but it is now part of a hidden zone, and there are no other effects requiring the card to remain revealed. You don't get to see it once it's back in their hand.