Android phone management via 3rd party apps. Are these as good as what the on-screen figures speak?

[Android Central Question]

Okay. Here it goes : I was running out of storage on my Android smartphone recently and was also experiencing occasional lag on its otherwise fluidic performance. Thus, I decided to download 'Clean Master' app to make some space. Just before installing the app, I deleted the cache data from the phone settings which was to the tune of ~600MB. But when I opened Clean Master and proceeded to clear the junk files from one of the options provided, I was stunned to see 10.1GB worth of junk still sitting on my phone! My question is, how do apps like CM dig up so much junk data which is apparently not detected by Android's own phone management system? Is it a mere gimmick or is there a catch to it?
Now some details about that humongous junk sitting on my phone...Out of 10.1GB, about 9GB belonged to a photo editing app called 'PicsArt' (more specifically to a sub-menu called "PicsArt image preview") Let me tell you, I've been using this app heavily for the last 2 years or so but ~9GB of junk still looks formidable. And more importantly, if there indeed was this much junk produced by PicsArt, why was it not detected by Android's inbuilt mechanism for phone management? I believe there's rather a catch to it than the gimmicks because once I cleaned all that junk via CM, it actually created that much extra space on the storage (as seen from the settings menu). Post removal, I had 41.30 GB used of 56.55 GB space.

My phone's specs are as follows :-
--> Qualcomm Snapdragon 820 chipset ; Quad-core CPU (2x2.15 GHz Kryo & 2x1.6 GHz Kryo) ; Adreno 530 GPU.
--> 64 GB internal storage (non-expandable), 6GB RAM.
--> Running Android 8.0 with Oxygen OS 5.0.2 (it's a quite light OS. Almost feels like stock Android so I don't think it would've caused any significant bloating)

 

[belodion]

What to you seems like junk seems like no more than files to the system. It cannot know that you regard them as junk.
This may help: https://forums.androidcentral.com/showthread.php?t=315804

 

[Rukbat]

The first thing you do with any app made by Cheetah is delete it! They don't know how Android works. (Or they know but don't care.) "Cleaning memory" is nonsense in Android. Windows wants as much free RAM as possible (and runs better with a lot of free RAM), Android wants as much RAM filled as possible - unused RAM is wasted RAM. It's like putting a spark plug into a diesel engine. Freeing RAM in Android just doesn't make sense. If Android needs more RAM to use for the app you're running, it kills an app that's in RAM. (Android apps have to keep their current states at all times, so when you bring that app back [it's in 'Recents'], Android runs it, telling it "you were running", and it picks up from where it left off.)

The other thing CM might be trying to delete is system cache. The system clears its own cache - and it's located in a different partition than apps, so full system cache (which won't happen) won't affect how apps run.

The problem is that some Android apps use functions in other apps, so the app that's sitting there and that CM "thinks" is junk, is actually needed by the app you're using. So CM "cleans" it out, your app causes it to reload, CM cleans it out ... wasting both time and battery.

With 6GB of RAM, you have at least a couple of years before games are written that need that much. (Most large games need more RAM than other apps.)

With 64GB of storage, unless you store a lot of music, pictures or videos on the phone, you have plenty of space. (8 full-length movies, in the wrong format, can completely fill that 64GB.) If you have to keep that much stuff with you, get a USB-C (I'm assuming a OnePlus 5 or 5T) adapter and a 128GB or 256GB microSD card. Then you have that much extra storage for videos, music, pictures, etc.

 

[Mooncatt]

As @Rukbat said, don't even bother with these types of apps. Not just Cheetah Mobile (which has a bad reputation for data mining), but anything similar. Anything that they can do is something you should already have the ability to do from your system settings, should you need them.

Generally, these functions are not needed, but sometimes an app goes rogue or something and you do need to do a little troubleshooting. Apps like Clean Master take the shotgun approach. Sure it may temporarily fix a lag problem or battery hog, but you'll never know what app was causing the problem. What you should do is analyze what's going on and try diagnostic steps like clearing app cache or force stopping it (which clears it from memory) on a one by one basis. This way you can hopefully find out the root problem and address it.