Hey, couldn't find this anywhere and was curious of the answers.

After each act it mentions that any quest the heroes did not attempt are considered to have ended in Overlord victory. What exactly does that mean?

An example, in the original quest (Journey's in the Dark) there are 2 quests in act 1 that come with relics, does the Overlord get them at the interlude since they ended in overlord victory?

Another example, in the Labyrinth of Ruins campaign, the power of the webs quest where she changes into the spider queen, does she get that if they go into the finale without attempting it? Or "Spligs revenge" overlord card if they don't attempt the quest in act 1?

I do not have LoR, but the statement about the other two quests not chosen being won by the OL refers to deciding which of the Act II quests gets played. The OL does not receive the rewards for the two quests that were not played and does not receive the relics. There is a quest reward in the first expansion that allows the OL to choose one of the unclaimed relics for the finale.

After each act it mentions that any quest the heroes did not attempt are considered to have ended in Overlord victory. What exactly does that mean?

It means that they count as OL victories for the purpose of picking Act II quests. ie: If "A Fat Goblin" was not played in Act I, then "The Frozen Spire" will be available to pick in Act II; "The Monster's Hoard" will not.

Nothing from the unplayed quest is awarded to anyone. It's just a way of defining what would otherwise be an undefined value when picking Act II quests.