package com.example.aplicao_zero;
import android.app.Activity;
import android.os.Bundle;
import android.widget.Toast;
import androidx.annotation.Nullable;
public class MainActivity extends Activity
{
    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Toast.makeText(this, "\n\nOlá, tudo bem??\n\n", Toast.LENGTH_LONG).show();
    }
}
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.aplicao_zero">
    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/Theme.Aplicação_zero" >
        <activity android:name=".MainActivity"> 
// esse '.MainActivity' fica sublinhado em vermelho e ao apertar F2 o problema descrito no titulo cai nele
        <intent-filter>
            <action android:name="android.intent.action.MAIN"/>
            <category android:name="android.intent.category.LAUNCHER"/>
        </intent-filter>
        </activity>
    </application>
</manifest>