Is there a particular word used to refer to them?
They are often called floral arches:
- 17 Epic Floral Arches and How to Get One
- 10 Darling Floral Arches for Your Wedding Ceremony
- The 3 Things You Need to Know About Floral Arches for Your Wedding
Also flower arch is used.
"Arbor" and "pergola" are terms for these types of structure. There are many web pages giving further information. One view is that an arbor has two or four legs (the vertical part touching the ground) whereas a pergola has four or more verticals. However many people assume the words are synonyms.