What form_with does

form_helper.rb

# helpers/form_helper.rb
def form_with(model: nil, scope: nil, url: nil)
    # ...
    url = polymorphic_path(model)
    html_options = html_options_for_form_with(url, model, **options)
    form_tag_html(html_options)
end

# helpers/form_tag_helper.rb
def form_tag_html(html_options)
  # ...
  tag(:form, html_options, true)
end

# helpers/tag_helper.rb
def tag(name = nil, options = nil)
  # ...
  "<#{name}#{tag_builder.tag_options(options, escape) if options}#{open ? ">" : " />"}".html_safe
end