This trick is dedicated to a very useful PCF NN DropDown
Today, the only solution with CRM to display a list of Many-to-Many relationship items into a form is actually to display it as a list.
You can also have a new button which will open a new form in a popup but let's be honest, it is not really user friendly.
That is where the PCF NN DropDown steps to the floor !
With it, you will be able to turn a Many-to-Many relationship into a multiselect dropdown.
The bonus is that with an input parameter, you can add a filter to the control to search your item !
It has been developed by Niels Minnee and it is available on pcf.gallery.
If you need to install it, the managed solution is available. But if you want to modify it, the sources are also available and you can regenerate managed or unmanaged solution as you wish. You can use this tutorial to help you doing it.