How do I fix this depression in my concrete patio?
Is this a drain strip?



If so it may be possible to cut some shallow grooves in the patio concrete decking to conduct the standing water in the depression over to the drain strip.



It would obviously be your choice but this may look better overall than a smoothed on patch or filler. Such grooves could be cut by someone experienced with using a portable power grinder with a thin cutoff type blade.