Are all Android apps device dependent ? Like for example if two different devices run the same version of Android can I interchange the app apk files ? Like the camera app ? Can I take the camera app of device A and install it in device B ?
Are the apps made by manufacturers or are they a part of the Android OS ? Like would it be possible for me to the get the Gallery apk of the Jellybean OS if I downloaded their SDK from Google ?
Not all apps are device dependent, but many are. One particular example is, indeed, the camera app, which is often dependent on OS revision, the actual hardware, and the framework the OEM is using (such as Touchwiz on a Samsung device). Many are reliant on the version of Android, although typically this only impacts using on an older version, up to a point.
As well, apps can be either developed separately or part of the system. Just depends on how the manufacturer wants to handle it. But they'll typically, these days, update them separately which then replaces the system app (really it just covers up the old version which remains), so the answer tends to be "a little of both." But if it's something made by the device manufacturer, they WILL lock it to their devices.
However your particular example, of the Gallery app from Jelly Bean, would probably at least partially function on a newer OS. That's based in AOSP, so not device-dependent. Only possible issue is whether it has any conflicts with newer apps on a newer OS.
Most of the things you as a user interact with on Android (including the home screen itself) are apps. Android comes with a set of apps created by Google that ensures a basic level of functionality (e.g. Clock, Home, Download manager, web browser, Dialer, Contacts manager, Calendar, SMS/messaging, etc.) These are generally non-removable by users. However, due to Android's open source nature, manufacturers and carriers are able to modify, or completely replace these apps with their own creations.
Apps supplied by the manufacturers are very device-dependent. Most depend on the particular framework that the manufacturer added to "stock" Android. For example, you cannot take Samsung's Camera app from a Galaxy S5 phone and have it work on an LG G3. Even if you manage to install it successfully, it will most likely not start up, or crash, because that app has dependencies on Samsung's TouchWiz framework and libraries that would not be present on LG's, Motorola's, or even stock Google's Nexus lines of devices.
3rd-party apps that you find on the Play Store are created to be compatible with as many devices as possible, because developers usually want as many people as possible to be able to buy and use their apps. You can generally "move" them between vastly different devices without issues, provided they are not restricted by by Android OS version running on the target device. Apps that make use of newer features that became available on certain Android versions will be incompatible with older OS versions. The Play Store app itself handles this, and will not even show you apps that are incompatible with your phone. You can still side-load such apps, but don't expect them to work well or be stable.
