jazakallha for liking, engine bay just cleaned still have to dressup with plastic / rubber trim restorer 
umar bhai very beautifully defined u apc, its function and usage
yes true we have mix apc with water and adjust its streangth as per usage as in neat condition its bit aggressive
as i stated above tools and products i used in this cleaning, i didnt wash engine with tab water
i am having differnt spray bottles ( easily avilable at metro and hyperstar ) so some bottles having water and some this apc mixture
i started from one corner of engine bay by spraying water and than sprayed apc, let a moment to apc to workout and then by help of differnt assorted brushes did some brushing and than again washed with water spray bottle and dry with microfiber
just dont be hurry and be patience as u have to deal some very complex corners in engine bay which u can left un done in hurry
after cleaning whole u have to apply rubber/plastic trim detailer to dress up
haha dear umar bhai i am not pro trust me so dont place me high
i will just add 2 small things here as u already describe whole process beautifully
1) u have to adjust the apc /water retio as per subject u going to workout e.g for engine metal parts i used retio 5:5 while for plastic and rubber parts 3:7
2) in one liter prepare apc mixture (including water) i can do 5 engine bays 
means 200ml apc mixture enough for one avg condition engine ( sure if engine in extreem dirty condition u have to increase qty )
here i give u exact tareeka-e-wardat 
work in cool place in shed
first spray water , this will not only washout loose dirt but also wet the surface and require less qty of apc mixture
after spraying apc wait a minut so apc workout than take a soft brush in one hand and in second take spray bottle contaning simple water and while brushing keep spray some qty of water , normally in one turn its work however for extreem dirty oily panel u can repeat the process
