The Android manifest file describes essential information about your app to the Android build tools, the Android operating system, and Google Play. Every Android app or unity app running on the Android platform must have a Manifest file. The manifest file contains information about package name, permissions, application name, icon, activities, services, providers and much more. Usually, Unity takes care of this manifest file generation and putting it in the right place but sometimes we need to add some custom tags to the final generated manifest file like adding intent-filters to the activity tag. We can add a custom manifest file and Unity will generate a merged manifest file at the end which will be shipped along with the final apk generated. We can copy a basic structure of the custom manifest file from /Temp/StagingArea/AndroidManifest.xml And can paste the modified AndroidManifest.xml at Assests/Plugins/Android/AndroidManifest.xml Please note that if this file doesn't exist for you, change the target platform to Android and build the Unity project once. It will generate the Temp folder and this file inside it. This is a temporary file that is created by Unity but we can copy it.
The AndroidManifest.xml for a new hello world project in unity looks like the below one but I will suggest you create your own and copy that instead of using the below one.
The final output file will be a merge of one created by Unity and the one we pasted at Assests/Plugins/Android/AndroidManifest.xml Now we can modify it according to our need like we can add intent-filters to it and that will be available in the final Manifest file generated by Unity.