You may use many plugins in Jenkins, and finding the code for those plugins to use in Jenkinsfile is sometimes challenging.
Declarative Directive Generator is helpful to generate the code to use in Jenkinsfile for most of the plugins and pipeline steps

You can generate code for both Scripted and Declarative Jenkisnfile

For Scripted Jenkinsfile use Snippet Generator and for Declarative Jenkisnfile use Declarative Directive Generator

Reference: Snippet Generator

How to go to Snippet Generator

In your Jenkins URL append path /pipeline-syntax E.g. https:jenkins.com/piepline-syntax

How to go to Declarative Directive Generator

In your Jenkins URL append path /directive-generator E.g. https:jenkins.com/directive-generator then click on Declarative Directive Generator

How to go to Declarative Directive Generator from Pipeline Page

From any pipeline page, you can click on Pipeline Syntax to go to Snippet Generator or Declarative Directive Generator

Generate code for cron triggers using Declarative Directive Generator

Go to your JENKINS_URL/directive-generator E.g. https:jenkins.com/directive-generator

Under Sample Directive choose triggers: Triggers from dropdown

Click on Add and choose cron: Build periodically

Enter the cron expression * * * * *

Click on the Generate Declarative Directive to generate the code

Copy the generated code

triggers {
  cron '* * * * *'
}

Generate code for tools block using Declarative Directive Generator

Choose tools: Tools from the dropdown under Sample Directive

Click on Add and choose maven: Maven

Click on the Generate Declarative Directive to generate the tools block code

Copy the generated code

tools {
  maven 'maven-3.8.8'
}

Generate code to trigger other pipelines using Declarative Directive Generator

This feature was available from scripted Jenkinsfile, so it needs to generate it from the Snippet Generator

Go to Snippet Generator, choose build: Build a job from Sample Step

Enter the child pipeline name 01-hello-world-java you want to trigger under Project to Build

Enable Wait for completion depends on your needs

Click on Generate Pipeline Script to generate the code to trigger the child pipeline

Copy the generated code

build '01-hello-world-java'

Like this you can generate the Jenkinsfile code snippet for most of the plugin use cases