When I pick up weapons/ shields, then on the right side of the screen, you see it, but with a red/green/gray arrow pointing up, down, and sideways. what does that mean? I can pick up a fresh new weapon, but it still has the red down arrow on it. Other times, it's green, but it's already used. What does that mean? I'm really confused.
That arrow is comparing the weapon's power or the shield's armor, not the durability. Weapon and shield effectiveness is not linked to durability, they will do or block the same amount of damage if they are brand new or one hit from breaking. This is why you see a green arrow even if a weapon is already used, because it's stronger than the weapon you have. The gray sideways arrow means the weapon/shield is equal to your equipment, but again ignores durability. I see it as saying what you've picked up is a "sidegrade" instead of an upgrade or downgrade.