What kind of physical phenomenon are thoughts? Are sensations thoughts?

What are qualia (a sensation or property thereof)? They are very real for us? Where do they belong in the real world phenomena? Are they not like a thought? Aren't thoughts the same kind of intangible thing?  Like a sentence? How are they different from a sentence? 0ne difference is thAt a sentence has an observable representation - the sounds we utter.

How do we form an internal representation from sensory input , and how do we trigger it again, so that we can think of something when it's not in front of us?