You use only as many of the trays as you need, and the sheet is for making fruit leather. The screens are for sticky things like sugary fruits that stick to the trays and are hard to get out - they’re flexible, so you can sort of peel off dehydrated pineapples, peaches, pears, etc. They’re also dishwasher safe, which your trays may not be. Finally, they are good for little leafy herby things that fall through the trays.
The “airflow” adjustment is actually a heat adjustor - many dehydrators actually get too hot, and opening that little panel lets excess heat out. How do you know if it’s too hot? Experience and smell. If it smells like yummy cooked food, it’s too hot. Often the lower trays closer to the heating element will get hotter (depite the fan, and despite what the manufacturers claim), so you might want to rotate your trays periodically throughout the drying time to even everything out.
Start with some easy stuff to try it out. Peel, core and slice an apple into 1/4 inch slices. Put the slices on the trays, stack 'em, and plug it in overnight. Check your apples. Are they dry and leathery yet? Then congratulations, you have dried apples! Still got some life in 'em? Give it another couple of hours. The total time anything takes depends on how many trays, how wet it was to start, how thickly it’s sliced, the temperature and fan on your unit and the ambient air humidity.
If you want to try fruit leather, whir up a bunch of pitted cherries in a blender with just enough apple juice to make it a paste. Put the plastic sheet on a tray, and carefully and slowly pour the cherry paste on. Let it dehydrate, peel and enjoy. You can buy more sheets online or at a health food store, but I use parchment paper, cut to fit.
You can grow and dry herbs, like oregano, thyme, marjoram, dill, etc. I’ll admit, I dry excess basil from my garden, though the flavor of dried basil isn’t as good as fresh. Just pick your herbs, rinse them in water if they need it, and lie them in a single layer on the trays. Really wet or fat leaves, like comfrey or mullein, do best if you flip them once or twice during drying.
Homemade beef jerky is a thing of art. There are dozens of recipes online, so I won’t bore you with the details. Homemade beef jerky is to storebought beef jerky as those little foil wrapped Easter “chocolates” to a fine Belgian dark chocolate dipped orange peel.
Just get in there and play with it. Most of what you try will work fine. Some of it won’t. But every dehydrator is different, and you just have to play with yours to see what it’s personality quirks are.