iMac 2015 27" i5 as an additional display
Can I connect the iMac 2015 27" i5 8GB as an external monitor via Thunderbolt adapters for Macbook Pro M2 Max 2023? or any other way?
Or will it work if I install windows OS instead?
Nope' both of those Mac's are too new to support Target Display mode via Thunderbolt.
carefully read > Use your iMac as a display with target display mode - Apple Support
There are only network based solutions like Duet Display and Luna Display.