How do I allow full HTML while rendering a view?
I would suggest having your view display "content" of type "teaser" which would then allow you to use a twig template file such as node--content-type--teaser.html.twig.

This way you can add whatever markup you want to the twig file... even iFrames.

If you must have fields, for instance in exposed filters combine, remember you can "force" use fields in views :)