All the advice seems to be to add a few inches of compost every fall to a raised bed, but no one mentions taking soil out; how can you just keep adding soil to a raised bed ...I mean surely it will fill up and then what? Should you dig half out and replace it or just a few inches ...which part of the bed will have the best soil, top middle or bottom. I can't find aany video or article that addresses this issue so I figure I'm missing something simple or obvious?
Compost is not "solid" material. It may be 40% water, and also contains a lot of air unless it has been compacted.
The organic material in the compost is gradually decomposed by micro-organisms into chemicals which are soluble in water and can be absorbed by the plants growing in the bed.
The chemical composition of compost is mainly carbon, hydrogen, oxygen, and nitrogen, all of which end up either in the plants you are growing, or in the air or ground water.
The only material which will never "disappear" is any inorganic material in the compost, and the only way that gets into compost at all is as part of the material that was "harvested" to create the compost pile - e.g. soil sticking to plant roots, etc.