Hi @Saad
Auto-send does not require ODK Collect to remain open. When a form is finalized while the device is offline, it is queued locally and Collect will send it when a suitable network connection becomes available. This is designed to work even when Collect is not running, so the field officer should not normally need to reopen the app.
Pending submissions should also resume automatically after a device restart (once the phone is unlocked), without opening Collect. Note that Android may defer background tasks to save battery, so submissions might not be sent the moment the connection returns.
One important exception is when Collect has been force-stopped. In that case, Android won't run its background tasks until the app is opened again. Some manufacturers (e.g. Xiaomi, Huawei, Oppo) apply aggressive battery optimizations with a similar effect, so on such devices it's worth disabling battery optimization for Collect.