I've turned off my iPhone. Then I turn it back on.
I have my phone connected to the macOS console. I filter by process:SomeAppName
And about 1 minute after reboot, I see app name appear with that filter.
Is that expected?
It would not “open all apps.” But certainly, it may open some apps (e.g., particularly certain “Background Modes” that are configured under a target’s “Capabilities”). See About the Background Execution Sequence, About the App Launch Sequence, Choosing Background Strategies for Your App, and WWDC 2020 Background execution demystified. None of these enumerate the specific cases where apps can be launched in the background automatically, but might be a starting point for your research.
FWIW, each of the target “Capabilities” » “Background Modes” has slightly different behaviors (i.e., some cause an app to be relaunched in the background triggered by some system event, others launch at the discretion of the OS, for example, if on wifi and/or when charging, and some gather data out-of-process and are delivered when the user launches the app). So you have to go through these individual “background capabilities”, one by one, and see which cause the app to be launched in the background and which do not.
But if an app has one or more background capabilities enabled, it could easily be relaunched. The “Background fetch” and “Background processing” are two prominent examples that can easily cause the app to be launched in the background without user intervention. The background location services can be configured either way (launch the app on location change vs. deliver location updates when the user next launches the app). It just varies for each of these background modes. As a general rule, Apple tries to offer out-of-process solutions where it can (to minimize power-draining launches of apps in the background), but provides methods to have the app launched in the background where needed.
In the past we would have concerned ourselves solely with apps being launched as a result of background capabilities. But, as About the App Launch Sequence says, iOS 15 complicates the “which apps might launch” question, as it has “prewarming”. So that might cause apps to launch in the background without user intervention, too.
You ask:
Would the app get killed/suspended after getting launched?
As a general rule, apps are never “killed”, unless you violate the contract of the background execution (e.g., fail to call the completion handler appropriate for that background service or block the main thread and get killed by the watchdog process). All of the various background modes have some mechanism to gracefully inform the OS that your background process is done and that the app can now be suspended again. But in those cases where the OS kills a misbehaving app, it generally no longer participates in future background execution, which is why it is so important to finish background processes gracefully.
Obviously, suspended apps can be evicted/jettisoned when memory is required by the OS. But this graceful termination is very different than an app being “killed.”
BTW, to clarify the observation that killed apps will not be launched by most background services, the old “App Programming Guide for iOS” gave us hints about how most background services will not be launched again, but that background location services might. The guide is no longer found on Apple’s site, but here is the relevant excerpt:
In most cases, the system does not relaunch apps after they are force quit by the user. One exception is location apps, which in iOS 8 and later are relaunched after being force quit by the user. In other cases, though, the user must launch the app explicitly or reboot the device before the app can be launched automatically into the background by the system. When password protection is enabled on the device, the system does not launch an app in the background before the user first unlocks the device.
The new documentation verifies that background location services “relaunch an app … even after the user force-quits your app.” But this older quote places that observation in a broader context.