I have a number of random potted plants in my apartment and a few of them are looking to be very tall. Specifically, orange and chili pepper. Once upon a time I replanted some of them and discovered that every plant has its roots concentrated tightly at the very bottom of their pots. It's probably where the most water accumulates, and naturally plants want it and whatever minerals it carries. Then I thought maybe I should raise the "root level" by lifting all the soil and roots from the pot, adding a few centimeters of soil and then putting the plant back in the pot. It should give the roots more space to grow downwards and allow to get more water (increased surface of roots). Does this make sense or will it harm the plant?
No, rearranging the roots won't do much for them. You can do one of two things. You can either repot them and trim any roots that won't fit, or repot them in a larger pot.
If you choose to keep them in the same pot, which is fine, you need to prune the roots so that they could potentially fit into the pot without bending and not touch the sides of the pot. When you put them, so will naturally fold up, but you judge how much to prune based on the size of the pot vs the root ball.
If you choose to pot in a larger pot, then you'll still need to trim excessively long roots. Some plants like to be root bound, like my peace lilies. They don't start blooming till they've refilled the pot with roots. However, I don't think the two you mentioned would like it.
Pot size is going to limit plant size to an extent, because it will limit root growth, and through it nutrient absorption and water intake. Some plants will actually burst their pots with their root growth, but for most of them, you'll notice a drastic difference in size in a potted plant vs the same one in the ground.
You also don't want to repot them into a huge pot, thinking it'll give them room to grow and have a lot more nutrients. The reason is because when you water it enough to saturate the pot, the plant will be too small to absorb enough water to allow oxygen to the roots and they'll rot. Even if you plan to end up with a large plant in a big pot, you need to gradually step the pot size up. You should let it grow in a particular pot till it seems to have out grown it, by becoming root bound, not showing growth, etc... Then move it one or two pot sizes bigger.
Another reason to repot once every year or two is not only because the soil looses nutrients, but it also becomes compacted, slowing root growth and preventing oxygen and water from getting to the roots.
Either way you go, I think you'll do okay. You shouldn't see any change for a while and may even see a little die back from transplant shock, but they should take off and start growing in a couple of months.
One last note is that you shouldn't expose the roots. I don't know if that's what you meant when you talked about raising it and putting some dirt in the bottom, but roots are used to the environment they grow in and a change will usually kill them. The plant will usually replace them with new roots by the time they die off, but if you expose any to the air once it's repotted, they'll die off. Good luck.