Can't Understand Deep link and Intent - html

Question

I want to use deep link to create a Online-Shop App. So Hear me out.
I know I have to use Intent and add <Intent-Filter><Intent-Filter/> tag in my AndroidManifest.xml file but I can't begin to understand how all of it works. There are few packages in pub.dev like flutter_branch_sdk: ^3.4.0 or receive_sharing_intent: ^1.4.5 but I'm hoping for a complete explanation or some sort of guide here. I appreciate the time you spend reading this in advance.
Here we go:
I have a app that has a Payment page. when user clicks on the Pay Now Button, He is transferred to a Safe Payment Gateway and interacts with the page. so there are 2 possible scenarios
He Succeeds in Completing the task and pays for the CARGO.
At this point Browser must Return to my app and gives me a text/message such as success and the app displays the Payment_Successful_Page
He Fails in Completing the task and pays for the CARGO
At this point Browser must Return to my app and gives me a text/message such as failure and the app displays the Payment_Failed_Page
That said If you know how it works maybe you can answer the questions below. (I have gathered the code parts from few documents and guides)
What is the <actvity><actvity/> do and what does the code below configure?
<activity
android:name=".MainActivity"
android:launchMode="singleTop"
android:theme="#style/LaunchTheme"
android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection|fontScale|screenLayout|density|uiMode"
android:hardwareAccelerated="true"
android:windowSoftInputMode="adjustResize">
android:launchMode="singleTask"
<intent-filter>
...etc categories
</intent-filter>
</activity>
what does the <action/> do? How should I use it in app?
<intent-filter android:autoVerify="true">
<action android:name="android.intent.action.VIEW" />
<action android:name="android.intent.action.BOOT_COMPLETED"/>
<action android:name="android.intent.action.INPUT_METHOD_CHANGED" />
<action android:name="android.intent.action.SEND" />
<action android:name="android.intent.action.SEND_MULTIPLE" />
<intent-filter />
how can I work with <category/> and What does the code below says?
what is the tag I should use for my scenario here?
<category android:name="android.intent.category.DEFAULT" />
<category android:name="android.intent.category.BROWSABLE" />
how can I work with <data/> and What does the code below says?
How to get that text/message I want from the browser?
How to configure my own website where "example.com" is?
What are scheme and host here and how should it be written? (with 'http://' or without)
<data android:scheme="myapp" android:host="example.com" />
<data android:scheme="https" />
<data android:mimeType="text/plain" />
<data android:mimeType="text/*" />
<data android:mimeType="video/*" />
<data android:mimeType="image/*" />
<data android:mimeType="*/*" />
<data android:scheme="http" android:host="flutterbooksample.com" />
<data android:scheme="https" />