How do I soundproof my apartment door?

Might try an egg crate mattress topper over a layer of batting, finished off with upholstery fabric. Finish the edges with either a wood frame or upholtry trim.

There is a 1 inch space between my apartment door and the floor. It lets cold or hot air in as the temp in the hall changes; I heard people passing my door and chatting. Assume they could hear me if they stopped to listen. I asked Mgmt. what could be done. They gave me (what luck?) a "thing" that has two rolls (like small pool noodles) attached by cloth. You slip it under the door. It fills the space on the outside & inside. Stopped the heat/cold issue and I don't hear people anymore. I'm sure if you are handy, you could make this item. They are not expensive to buy. I checked after I was "gifted" one by management. Good luck.

That is a Cheap Hollow door for you.

Not only can you hear what's outside...what's outside can hear inside too.

I understand your privacy concerns.

Best thing you can do to correct this Privacy issue is Drill a Hole in the Panels and Spray in Foam insulation.

That is truly the only option you have.

You may have to hang quilts or framed sections of cork on your walls and doors, in addition to purchasing the "draft dodger" to place under your door. Adding a Flokati rug or similar will help absorb sound, as will upholstered furniture, thicker, quilted bed linens, bath rug, cushioned kitchen mats, heavily lined floor to ceiling draperies, etc.