Nested forms are forms that handle nested models and attributes in one form; e.g. a project with its tasks or an invoice with its line items.

Before Rails 6, Cocoon is a good choice for creating dynamic nested forms. But Cocoon needs jQuery to work well, it's a very old library on modern-day frontend frameworks.
When Stimulus is came out, Rails devs is suggested to use Stimulus as Javascript library in their projects. So, I created a gem for handling dynamic nested forms with Stimulus JS.

$ bundle add rondo_form

$ gem 'rondo_form', '~> 0.2.6'

$ rails g rondo_form:install

rails g scaffold Project name:string description:string
rails g model Task description:string done:boolean project:belongs_to

class Project < ApplicationRecord
  has_many :tasks, dependent: :destroy
  accepts_nested_attributes_for :tasks, reject_if: :all_blank, allow_destroy: true
end

<%= simple_form_for(@project) do |f| %>
  <div class="form-inputs">
    <%= f.input :name %>
    <%= f.input :description %>
  </div

Rondo Form is easy to use, it has the same tag helpers name as cocoon: link_to_add_association, link_to_remove_association.
This gem does not need JS dependencies to work with, when you run under command, it will generate nested_rondo_controller.js in your app/javascript/controllers/ folder:

rails g rondo_form:install 

And auto import this controller into index.js.

 js
import NestedRondoController from "./nested_rondo_controller"
application.register("nested-rondo", NestedRondoController)

You must add data-controller="nested-rondo" to an element, that wraps fields_for and link_to_add_association helper.
For example, we have Project model, which has has_many relationship with Task model.
In your projects/_form partial:

 erb
<%= simple_form_for(@project) do |f| %>
  <div class="form-inputs">
    <%= f.input :name %>
    <%= f.input :description %>
  </div>
  <h3>Tasks</h3>
  <div data-controller="nested-rondo">
    <%= f.simple_fields_for :tasks do |task| %>
      <%= render "task_fields", f: task %>
    <% end %>
    <div class="links">
      <%= link_to_add_association "Add Task", f, :tasks %>
    </div>
  </div>
  <%= f.button :submit %>
<% end %>

In your _task_fields partial:

 erb
<div class="nested-fields">
  <%= f.input :description %>
  <%= f.input :done, as: :boolean %>
  <%= link_to_remove_association "Remove Task", f %>
</div>

This sample I built with SimpleForm gem, but the idea is the same with Rails standard form.