After this suggestion of Gerald, "So you can still add a xml folder under Resources (under the Android folder that is)", I finally got it!

Add a network_security_config.xml file, under the Android folder, with:

<?xml version="1.0" encoding="utf-8"?>
<network-security-config>
    <base-config>
        <trust-anchors>
        <!-- Trust preinstalled CAs -->
        <certificates src="system" />
        <!-- Additionaly trusted user added CAs -->
        <certificates src="user"/>
    </trust-anchors>
    </base-config>
</network-security-config>

And in your AndroidManifest.xml add the: android:networkSecurityConfig="@xml/network_security_config" attribute to your Application node.
So, add it to the already existing node, don't add a new one (or else you'll get strange errors):

And the trust anchor exception is gone, but I'm still curious if there is another way - without the network-security-config. ;-)

PS Don't forget to add the CA certificates to the Android device, as stated in the original question.

OK so I see a couple of things here and not sure what you're after exactly so let me go over it one by one.

Let's start with: if you can, please avoid using clear text traffic! ;)

UseClearTextTraffic Attribute

Then, the easy one, you want the [assembly: Application(UsesCleartextTraffic = true)] oneliner. You can totally still do that and actually you can now that throughout the whole project I think. But it makes the most sense in Android.

Notice how the attribute says assembly so it works for the whole assembly anyway and it doesn't really matter where you put it. That is how it typically works. However, in .NET MAUI there is already a [Application] attribute above the MainApplication, so open that and modify it like below.

namespace MauiAndroidClearText;

[Application(UsesCleartextTraffic = true)]
public class MainApplication : MauiApplication
{
   // Your code
}

Network Security Config

Basically your separate Android project is now under the Platforms\Android folder. Everything you put there, even if it's not there by default, will still behave as it was in a separate Android project.

So you can still add a xml folder under Resources (under the Android folder that is), then add the network_security_config.xml file with:

<?xml version="1.0" encoding="utf-8"?>
<network-security-config>
  <domain-config cleartextTrafficPermitted="true">
    <domain includeSubdomains="true">10.0.2.2</domain> <!-- Debug port -->
    <domain includeSubdomains="true">xamarin.com</domain>
  </domain-config>
</network-security-config>

And in your AndroidManifest.xml add the android:networkSecurityConfig="@xml/network_security_config" attribute to your Application node.

See a full sample here: https://github.com/jfversluis/MauiAndroidClearText

How you asked how you could trust a self signed root certificate (self signed CA) without needed to install in the device, I will share the code.

I am using a MAUI project that use .NET 6.

In the MAUI project, in platform/android/resources/raw, you have to copy your ca.crt certificate. It can have .crt or .pem extension. In my case, I use .crt.

The raw folder is not created by default, so you have to create this folder.

In platform/android/resources, you have to create the folder xml, so you will have platform/android/resources/xml folder. In this folder, add a new xml file, with this name: network_security_config.xml.

This file set the certificates in which you want to trust.

The code of this file is:

<?xml version="1.0" encoding="utf-8"?>
<network-security-config>
  <domain-config>
    <!--NOTA: el subdominio es necesario porque si no no se podrá instalar-->
    <domain includeSubdomains="true">192.168.1.200</domain>
    <trust-anchors>
      <certificates src="@raw/ca"/>
    </trust-anchors>
  </domain-config>
</network-security-config>

The certificate is ca, with no extension, if not, it will give an error when you try to compile.

You have to include a subdomain, in my case, the IP of the server, because I am not inside a domain. If you don't set this, when you try to debug it doesn't start, gives problems.

You have to edit the file AndroidManifest.xml, this exists. The code of my file is this:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android">
    <application android:allowBackup="true" android:icon="@mipmap/appicon" android:roundIcon="@mipmap/appicon_round" android:supportsRtl="true"
               android:networkSecurityConfig="@xml/network_security_config"></application>
    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
    <uses-permission android:name="android.permission.INTERNET" />
</manifest>

You only need to add android:networkSecurityConfig="@xml/network_security_config" in the application entry. This tells that you will config the network security.

That's all, you can build the project and then the application will include the CA in the trusted certificates, so you don't need to install the certificate in the device.

You can also see the following documentation for more information:

https://developer.android.com/training/articles/security-config

If you have some question, tell me and I will try tohelp.