What is a Jenkinsfile and what is Declarative vs Scripted Pipeline syntax?

pipeline {
    // Where to run: any available agent/node
    agent any

    // Environment variables
    environment {
        APP_ENV  = 'staging'
        MAVEN_OPTS = '-Xmx512m'
    }

    // Optional: tools to auto-install
    tools {
        maven 'Maven-3.9'
        jdk   'JDK-17'
    }

    stages {
        stage('Checkout') {
            steps {
                checkout scm  // checks out the branch that triggered the build
            }
        }

        stage('Build') {
            steps {
                sh 'mvn clean package -DskipTests'
            }
        }

        stage('Run Automation Tests') {
            steps {
                sh 'mvn test -Dbrowser=chrome -Denv=${APP_ENV}'
            }
            post {
                always {
                    // Publish JUnit test results
                    junit 'target/surefire-reports/*.xml'
                    // Publish HTML report
                    publishHTML([
                        reportDir:   'target/extent-reports',
                        reportFiles: 'index.html',
                        reportName:  'Test Report',
                    ])
                }
            }
        }

        stage('Deploy') {
            when {
                branch 'main'             // only deploy on main branch
                expression { currentBuild.result == null || currentBuild.result == 'SUCCESS' }
            }
            steps {
                sh './deploy.sh ${APP_ENV}'
            }
        }
    }

    post {
        success { echo 'All stages passed!' }
        failure {
            mail(
                to:      'qa-team@company.com',
                subject: "BUILD FAILED: ${env.JOB_NAME} #${env.BUILD_NUMBER}",
                body:    "Check: ${env.BUILD_URL}"
            )
        }
        always {
            cleanWs()  // clean workspace after every build
        }
    }
}

node {
    def mavenHome = tool 'Maven-3.9'

    stage('Checkout') {
        checkout scm
    }

    stage('Build') {
        sh "${mavenHome}/bin/mvn clean package"
    }

    stage('Test') {
        try {
            sh "${mavenHome}/bin/mvn test"
        } catch (Exception e) {
            currentBuild.result = 'UNSTABLE'
            echo "Tests failed: ${e.message}"
        } finally {
            junit 'target/surefire-reports/*.xml'
        }
    }

    if (env.BRANCH_NAME == 'main') {
        stage('Deploy') {
            sh './deploy.sh'
        }
    }
}