u/Away_Engine_1375

I had an hvac company do a annual service on my old (probably 25 years) unit. I watched out of curiosity. He opened the unit, sprayed the coils and dropped tablets. I started watching YouTube and saw much more extensive coil cleaning, specifically on the inside of the coil triangle. I opened the unit to try and attempt this but because of the coil orientation it appears I can only access the interior coils by removing the large intake duct (filter is in ceiling at beginning of duct). Is this correct? Do I risk anything by attempting to remove the large intake? Also I removed both side pieces and there was no obvious removable cover plate over the coils.