"Pick up" is a broad term, but is often done directly with the hand and fingers. "Scoop up" suggests using a scoop (an implement like a shovel). So "to scoop something up" is one specific way of picking it up. People don't want to touch dog mess, and find ways to pick up dog mess without touching it. A common way is to use a scoop made for the purpose. (In the United States, they are commonly called "pooper-scoopers" and laws requiring owners to pick up after their dogs are often called "pooper-scooper laws.") So, the verb commonly used with dog mess is "scoop." "Scooping" is a relatively imprecise, and when using a scoop to pick something up, it is common to pick up additional things along with it. So, figuratively, "to scoop" can mean to collect or gather in a wide-ranging, indiscriminate way. "I only had five minutes before the library closed so I just ran into the stacks and scooped up every book that had the right Dewey decimal number..."

The difference is more about ease of movement than speed. Both 'pick up' and 'scoop up' can be quick, but 'scoop' suggests a single continuous, fluid movement. 'Pick up' does not have this connotation of smoothness. When you scoop, it's a smooth, swift curve - like dipping a scoop into a tub of soft ice-cream and bringing it out again in one single, streamlined action. We use 'scoop' when we are literally using a scoop to pick something up, such as dog mess, but we also use it in a figurative sense. For example, you might say 'Jack scooped up all the prizes at the award ceremony'. This suggests that he won them all without any effort. Or you might say, 'I'll scoop you up from the station on my way home', suggesting that it's a quick and easy job - I'll swing by the station, you can jump in the car and then I'll carry on home. Does that make sense?

Good question! Saying "Pick up your dog mess!" has the same general meaning as saying "Scoop up your dog mess!" in that it's an order to clean up after the dog. However, the precise meaning is slightly different. To scoop up is to take something up either with a scoop or in a scoop-like motion. To pick up is to take something up with your hands or an instrument, in a similar motion to picking fruit.