This is how I use a date_select or datetime_select helper in a Rails ERB view with Bootstrap classes, so it looks Bootstrappy but also all on one line.

rails_date_select_bootstrap.html.erb

<div class='mb-3'>
  <%= form.label :birthday, class: 'form-label' %>

  <div>
    <%= form.date_select :birthday,
                         {
                           prompt:            true,
                           add_month_numbers: true,
                           end_year:          Time.now.year - 111,
                           start_year:        Time.now.year - 12,
                         },
                         { class: 'form-select form-select-lg d-inline w-auto' } %>
  </div>
</div>