advice on routine maintainance on iMac (2011 High Sierra
Advice on routine maintenance for 2011 iMac (High Sierra)
Do you mean cleaning the exterior?
How to clean your Apple products - Apple Support
Do you mean clean up storage space?
How to free up storage space on your Mac - Apple Support
Do you mean operating temperature?
Keep your Mac notebook within acceptable operating temperatures - Apple Support
Do you mean keeping it free of malware?
Effective defenses against malware and ot… - Apple Community
Do you mean maintaining a backup of your data?
Back up your Mac with Time Machine - Apple Support
Do you mean Optimize battery performance?
Batteries - Maximizing Performance - Apple
Be more specific with regards to what your definition of "routine maintenance" is. There is no "routinne maintenance" like there is for a car such as changing the oil.