When you interact with the door, a UI will pop up and have you select which difficulty you can play, select a key to use then you can add a raid mod and open the portal. As you enter the door, you'll be teleported to the waiting area, giving other people a chance to join your raid, which will also increase your chances for loot by a set percentage. Loot that has a beam of light above it is owned by you, if it doesn't have a beam of light, then it isn't yours. There are several different resources you can collect in-raid. You can collect Copper, Iron, Gold, Diamond, and Berries. Here's an example of an ore node, and a berry bush. Killing the Boss(es) on Wave 5 drops you Stat Mutators. Killing the Boss(es) on Wave 10 drops you a key shard respective to the tier of raid. Both Bosses will drop you a potion and possibly more when you kill them!
You can take 10 medical items and 10 potions with you in a raid. The type of loot which you earn is based on the difficulty of the raid- see the table below regarding difficulties you need to play to have a chance of getting higher grade weapons.