CPU and RAM usage statistics for iPhone/iOS (similar to Activity Monitor)?
I use CPU-x Dasher for this. It displays :

CPU usage, frequency time
RAM usage (free/used/reserved…)
Benchmark took (not that much features but it’s still a benchmark)
Networks/Sensors data
And all of this can be integrated on a widget

It’s still limited compared to some Android apps and it has a lot of ads but it’s the best I could find yet.