Dominion: How exactly do cards "enter play"
Quotes from the Q in italics and marked with &gt;&gt;, other quotes are from the rules.

&gt;&gt; as you play actions, the go from being &quot;on your hand&quot; to being &quot;in play&quot;

Yes. Second edition rules, p. 4:

Playing an Action card has three steps: announcing it; moving it to the &quot;in
play&quot; area - the table space in front of you; and following the instructions on it

Same for Treasures, see below.


&gt;&gt;  at any time before your buying phase, you can lay down money cards, therefore also moving them from &quot;on the hand&quot; to &quot;in play&quot;,

Playing Treasures happens during the Buy phase (but has to be done before actually buying something). You can't do it in the middle of the Action phase, so definitely not &quot;at any time&quot;. This matters very much for the Actions that say &quot;discard your hand&quot;. p. 5:

Buy Phase: First you can play any number of Treasure cards from your hand, in any order. [...] You play one by moving it to the &quot;in play&quot; area;

There's nothing in the rules about playing Treasures during the Action phase.


&gt;&gt;  Also, it is my understanding that any action card that refers to &quot;cards in play&quot; only takes into account those cards you already moved from &quot;your hand&quot; to &quot;in play&quot;

Yes, &quot;in play&quot; is explicitly stated as a distinct &quot;area&quot; for the cards to exist. It also matters if you have to reshuffle mid-turn: discarded cards get reshuffled, cards &quot;in play&quot; do not.
Note that the Action or Night card you just played is also &quot;in-play&quot; when it is processed. This follows from the sequence described above, and is reinforced by the FAQ for e.g. Magic Lamp and Raider (also at the back of the Nocturne rules).


&gt;&gt;  meaning that it matters whether you place your money before or after playing an action card and that there is no fixed time when you play your money (i.e. before or after playing an action). Is that also correct?

No, see above.


&gt;&gt; Furthermore, as the night cards are played after the buying phase, all actions that were played during your turn and any money played before your buying phase is considered to be &quot;in play&quot;. Is that also correct?

Yes, except that the money is played during your buy phase. The Night step happens before Clean-up, which is where the &quot;in play&quot; cards are put to the discard pile. Nocturne rules:

Nocturne adds Night cards and the Night phase. In games using Night cards, the Night phase happens after the Buy phase - it goes, Action, Buy, Night, Clean-up.



&gt;&gt;  After the night phase, any card &quot;in play&quot; goes to the discard pile (except for duration cards that will be kept for the next turn).
&gt;&gt;  Any card not having been played, remains &quot;on the hand&quot; and goes directly to the discard pile, without ever entering play.

Yes. The rules don't say anything about the cards going &quot;in-play&quot; in between. (And why would they, they were never played.) Second ed. rules, p. 5:

Clean-up Phase: Take all of the cards you have in play (both Actions and Treasures), and any remaining cards in
your hand, and put them all into your discard pile.

Nocturne, p. 4:

Nocturne has Duration cards, [...] Duration cards are not discarded in Clean-up if they have something left to do; they stay in play until the Clean-up of the last turn that they do something.



&gt;&gt; However, the main point of contention/confusion is if there is any way of bringing &quot;point and/or curse cards&quot; into play, or do they always stay &quot;on your hand&quot; and go straight to the discard pile?

Unless there's a rule or a card that allows playing them, then no, you can't play them. The easiest way to get a Victory card &quot;in-play&quot; is probably the dual Victory/Action and Victory/Treasure cards from Intrigue (e.g. Harem), but that of course only works
because they're also Actions or Treasure. (I can't think of any ways to play them purely through being Victory cards, but that doesn't prove there isn't one.)