Develop Reference

When program is run the email app do not opens on mobile phone. OR when tried to modify code the app crashes

the email app is not opening on my android device. there are no error in code but don't know why app isn't working

You are using an old or deprecated way to start activity for ACTION_SEND scenarios. Use below snippet for the correct way.
val sendIntent: Intent = Intent().apply {
action = Intent.ACTION_SEND
putExtra(Intent.EXTRA_TEXT, "This is my text to send.")
type = "text/plain"
}
val shareIntent = Intent.createChooser(sendIntent, null)
startActivity(shareIntent)
You can check out the code snippet and detailed explanation here.
https://developer.android.com/training/sharing/send#using-android-system-sharesheet

Starting with API level 30, if you're targeting that version or higher, your app cannot see, or directly interact with, most external packages without explicitly requesting allowance.
Check this answer, it works for me.
https://stackoverflow.com/a/65166064/7248394

Making API calls while app is killed/not actively running

So I am making an app that has to check for some information, but I need to be able to do that while the app is killed/not actively running. So basically like Youtube's notifications or something. I am a beginner and watched some tutorials on FCM, which can send a notification while the app is killed, which is fine. However, I need to be able to periodically make API calls, check if a certain condition is true and send the notification if so (all of that while app is not running). I tried googling that and found nothing that can help me. So... any ideas? (No code included since I don't think it's relevant.)

If this condition only affects the display of notifications, you can simply check the condition when you receive a notification through FCM. Here's an example :
class FCMService : FirebaseMessagingService()
{
override fun onMessageReceived(message: RemoteMessage)
{
val myCondition = getConditionFromAPI()
if (myCondition) {
showNotification(message)
}
}
}

Use AlarmManager to request periodic runs of your code. It may be set up to broadcast a message time to time to your receiver (declared in AndroidManifest.xml or in your code).
It will work while device is on. To continue after reboot, you have to set up a receiver for Intent.ACTION_BOOT_COMPLETED and register it in your code (not in AndroidManifest.xml, as it no longer works in latest versions of Android).

Android: How to make notification visible even after app is cleared from memory

I want to make a notification which cannot be cancelled by the user manually, just like google navigation notification. I don't know how to do it.
Please Help.
Below is my code that I had tried.
mBuilder.setSmallIcon(R.mipmap.ic_launcher)
.setOngoing(true)
.setContentIntent(pendIntent)
.setContent(mRemoteViews)
.setPriority(0)
.setTicker(ticker);
Thanks.

Try adding this
mBuilder.setOngoing(true)
Other option is to use intentService or background service to make your notification for you instead of the application context.

Running Android App in background or as a constant notification

I'm learning how to make apps for android and I have started by creating one which makes my phone scream when its dropped.
I got it working to where the phone screams when dropped, but now I need to make it so that the phone screams when dropped even when the app is closed, and to show a notification in the notification bar saying that its running
What should I use to do this? Should I use intentService? Ive been looking all over and I'm not sure where to look. Any guides would be appreciated

You need to make your service run in the foreground. You can achieve this by showing a notification when your service is running.
This is how you need to make your service run as foreground
private void showNotification(String title)
{
NotificationCompat.Builder mBuilder =
new NotificationCompat.Builder(this)
.setSmallIcon(R.drawable.ic_launcher)
.setContentTitle(title);
startForeground(1000,mBuilder.build()); // 1000 - is Id for the notification
}
You can also set your custom RemoteViews in notification using setContent
You can remove the service from foreground state using stopForeground

check Service Training.
For your use case it's important that it's an foreground service:
Documentation Service

Android SMS API

I know that the SMS content provider is not part of the public API (at least not documented), but if I understand correctly it's still possible to use many of the SMS features as long as you know how to use the API(?).
E.g it's pretty straightforward to insert an SMS into your inbox:
ContentValues values = new ContentValues();
values.put("address", "+457014921911");
contentResolver.insert(Uri.parse("content://sms"), values);
Unfortunately this does not trigger the standard "new-SMS-in-your-inbox" notification. Is it possible to trigger this manually?
Edit: AFAIK the "standard mail application (Messaging)" in Android is listening for incoming SMSes using the android.permission.RECEIVE_SMS permission. And then, when a new SMS has arrived, a status bar notification is inserted with a "special" notification id.
So one solution to my problem (stated above) could be to find, and send the correct broadcast intent; something like "NEW SMS HAS ARRIVED"-intent.
Edit: Downloaded a third party messaging application (chompsms) from Android market. This application satisfies my needs better. When i execute the code above the chompsms notice the new sms and shows the "standard status bar notification". So I would say that the standard Android Messaging application is not detecting sms properly? Or am I wrong?

Unfortunately the code responsible for these notifications is hidden in the messaging application. The class MessagingNotification has a static method updateAllNotifications that you could call using a PathClassLoader and reflection:
PathClassLoader c = new PathClassLoader("/system/app/Mms.apk", getClassLoader());
Class.forName("com.android.mms.util.ContactInfoCache", true, c)
.getMethod("init", Context.class).invoke(null, context);
Class.forName("com.android.mms.transaction.MessagingNotification", true, c)
.getMethod("updateAllNotifications", Context.class).invoke(null, context);
This is obviously a very bad idea for several reasons but I can't think of another way to do what you described.

Could you trigger a PUSH notification after the SMS?
Thread: Does Android support near real time push notification?

Maybe you should replace
content://sms
with
content://sms/inbox