Pick up a backpack with E and press I or Tab to open it. The window uses a rusted, post-apocalypse look. Select a slot to inspect it, then drop one item or the whole stack. Anything you drop can be picked up again.
Your character crouches while you browse, and the world keeps running, so opening your bag in the open is a risk.
The same system powers the weapon wheel's ammo and grenade counts, which also show on the HUD.
What it does not do yet is consumables, so food and medicine have no effect when used, and your inventory is not saved between sessions. Both are on the list.