This The Last of Us 2 Walkthrough Chapter 2: Seattle Day 1 - The Theater explains how to get the power back on and highlights all the collectible locations including: 4 Artifacts and 2 Trading Cards.
The Theater Walkthrough
Check out the bathroom on the first floor and grab the supplements.
Artifact - Program for CassandraCheck out the crate to the left of the mannequin for an artifact in the form of a program.
Trading Card - Mortem
Next to the bathrooms on the main floor lobby is an empty concession stand area. Hop over the counter and start breaking the large class cabinets against the back wall, and look at the bottom of the one on the right to find a trading card tucked along the bottom.
There’s a locked door ahead but you’ll need a key to open it.
Artifact - Lone FEDRA Solider Journal #1
2nd floor: Go upstairs, to the left near the balcony is a makeshift tent and table with an artifact: Lone FEDRA Soldier Journal #1
Trading Card - Beyond
2nd floor: Walk past the movie posters and right, to the end of the hall. Check the sofa, overturned sofa, and stack of boxes and you’ll see a trading card underneath.
Go up the stairs to the 3rd floor and turn to the right. Climb out the window, up the ladder to the roof, and follow the rope to the generator you need to turn on. This area has scraps
Return to the 3rd floor to go back inside and walk straight ahead to the projector room ahead.
Artifact - Lone FEDRA Soldier Journal #2
Approach the table and pick up the document on top of the radio.Interact with the radio to get the keys for downstairs (they’re under the radio and will be revealed after Ellie hits it in frustration).
Return to the first floor and open the locked door.
Artifact - The Sick Habit Flyer / Set List
In the main theater, walk to the right of the stage. Pick up the artifact on the supply trunks: The Sick Habit Flyer / Set List.
Interact with the guitar case and a cutscene will play and lead you into the next section: Seattle Day 1 - The Birthday Gift.