Question #3- "Where do you plant-ground or grow boxes?"
Both. It depends upon the variety. All varieties can be ground in the ground, but some lend themselves well to container gardening too. What's the difference? Some varieties will develop tubers right under the mother plant. These can be grown in containers. Other varieties don't do that. Plant the wrong variety in a container and you're won't get any tubers. I have some sweets growing in my pallet grow boxes. A gardener on the top of Oceanview has success growing them in smaller containers (half barrels and old coolers).
From what I've read, ancient Hawaiians grew at least some varieties that produced their tubers right under the mother plant. They may have had other more far ranging types, but I've only read accounts of them making stone rimmed beds for sweet potato growing, and also making a puka (scooped out hole) in the ground and preparing it for sweet potatoes. Since they had dozens, possibly hundreds, of cultivars, I wouldn't be surprised that they had all the variations that we see today in modern cultivars.
Question #4- "How far from the main plant are the potatoes located?"
That depends upon the variety. Some produce all their tubers right under the mother plant. On the other extreme, there are types that never produce tubers under the mother plant. Most tubers are within a few feet of the mother plant, but I tried one variety once where I would find tubers up to 15-20 feet away! The neatest growing variety I had once was a bush type with 3-5 foot long vines and which produced lots of tubers only right under the mother plant. It was an ok sweet potato for eating but it wasn't outstanding. But it was easy to grow and very productive. Sadly I lost the variety when my neighbor's cows destroyed the garden. They ate every bit of vine and tuber! I've never been able to find that variety again.
Question #5- "How do you know when they are ready?"
That depends upon the variety. Some will be ready by 3 1/2 months from the time they were planted (for small tubers), others not until close to a year. For most varieties, check at about 5 months and see what has developed. If tubers aren't big enough, wait another 1-2 months and check again. If I know a variety's habit, then I can simply mark a date on the calendar for harvesting. But of course that date will vary a little due to the weather and time of year. They are slower to form good tubers during the winter and during drought.
Tubers can be harvested at any size. I don't need to wait for tubers to get big. In fact, I prefer the smaller sized ones myself. I'll sell the larger ones. Some varieties tend to produce fibers in the tubers if they grow too slowly or get too mature. Thus another good reason for harvesting young tubers.
"D" says that she's growing a purple variety and wanted to know when to harvest them. Not knowing which variety it is, I can only suggest that she do a little gentle digging and take a look-see. If it is an Okinawan type, I wouldn't expect harvestable tubers until 9-12 months. It's a very slow variety. But there are numerous purple types, some of which are ready at 5 months.
She also asked if there was visual clue for when the tubers are ready. While there might be for some particular varieties, I haven't noticed them yet. Some varieties will flower, most do not. Some grow long vines which slow down in growth, others don't. Some start producing smaller leaves, some don't. I haven't seen anything consistent as an indicator. If there is one, I'd like to know about it myself.