All will become more clear when you do your course ... psi for wand can be from 200 to 400 ....psi for upholstery depends a lot on the tool your using but generally lower . 100 to 250
If you look on youtube you might get a few tips for now ... carpet cleaning is a bit like USA Tims video here ( dont use as much prespray as him on a regular carpet )
chemicals ... nobody might answer your question cause there are so many brands to choose from , each have their own favourites . In general think of cleaning chemicals like this ... there are as few as 10 or 15 chemical indredients used in common by all the product brands , a particular brands product might contain three or four ingredients from that list to form a product aimed at a particular use ... like in the video ... the product that fizzes a bit had an oxy booster in it ( a bit like vanish oxy laundry booster ) the other product had a citrus solvent ( there are UK products similar to these )
https://www.youtube.com/watch?v=Gez23otdM_k&t=519s