Is uninstalling apps on iOS thorough? first of all, my as accurate as possible definition for &quot;thorough&quot;:

No executables (binary, scripts etc.) left on the iOS device
The application itself and the provider of the application has no way of utilizing the computing power of the iOS device (browsing the homepage of the app provider in a browser doesn't count, apparently)
Configuration files need not to be cleaned, and can stay unless these configuration files somehow can be executed on the iOS device

And this is going to be a multi-faced question so a rather comprehensive answer is expected but not required, feel free to give your input.
I'm from one of the Asian countries that are notorious for censoring internet access and monitoring citizens' digital devices.
Because of work I have to install several iOS apps that is developed within my country, and it is pretty much open secret that these apps monitor user behaviors and collect privacy data. The former employees say it out loud on social media without qualms, the CTO made it plain that they cooperate with the authority and news reports every now and then reveal people (usually criminals but who knows) get caught  because the police simply pull the chat log stored on a server somewhere.
I plan to migrate all these apps to another phone, and uninstall them from my iOS device so I could chat more freely with my love interest, (so xe could send me risque pictures, there, I said it)
So here's the question, is the unintallment thorough? Will apps leave binaries, scripts and other executables on my device, even if I uninstalled the app?
My phone is genuine, from original manufacture, not rooted or jailbroken at all
I understand if it's linux core OS we are talking about I probably shouldn't worry all that much but iOS is basically a closed-source BSD system, I want to be sure.
If you think there are exploits and flaws in the iOS itself and could be exploited by apps, feel free to inform me also, much appreciated.
Edit:I wasn't talking about iOS itself, I have confidence in it (else I wouldn't buy an iPhone to begin with), I'm talking about whether APPs could find exploits to continue running/semi-running on the device. I hope I make sense.
Yes, app uninstallation on iOS is very thorough. The app is in a normal circumstance not able to leave behind executable bits and pieces to infiltrate your phone, spend resources or otherwise change the behavior of the phone.
Note that the app might leave data behind in the iCloud, in your Keychain and other places - but it is data, not executables.
The reason I write &quot;normal circumstances&quot; is that you're considering app made by national states for the express purpose of spying. In that case, it is possible that the app exploits some kind of vulnerability in iOS that allows it to alter your system permanently. This would have to be done without somehow alerting Apple to it in their reviews. I wouldn't consider it likely for almost any app though.