The Note from mom is a letter from Ellie's mother, Anna, and an automatic artifact in Ellie's backpack at the start of Lakeside Resort. You can read both sides of it from the Artifacts menu.
Anna writes to her newborn daughter with dry honesty about not liking kids, then admits holding Ellie amazes her. She names Marlene as the person she trusts to raise Ellie, tells her the world is hard but worth living in, and urges her to find a purpose and fight for it.
Anna wrote it knowing she would not live long, which explains Marlene's role in Ellie's life. Ellie's reply when you flip the note shows how much she wants to live up to it.
Braun's Firefly Pendant
Braun's Firefly pendant rests on a bench inside a gazebo next to the first cabin at the resort, in The Hunt section of Lakeside Resort. It belonged to Paul D. Braun.
Ellie's backpack
Ellie's backpack is an automatic artifact Joel picks up in the back of the butcher shop in Lakeside Resort. As he walks in, he grabs it from a low shelf, and it stays in his Artifacts menu from then on.
Joel & Sarah photo
The Joel & Sarah photo is a picture of Joel with his daughter, an automatic artifact in Ellie's backpack at the start of Lakeside Resort. You can view it from the Artifacts menu while playing as Ellie.
Kristof's Firefly Pendant
Kristof's Firefly pendant is inside a blue storage container in the coal mine during The Hunt section of Lakeside Resort. It belonged to Travis Kristof.