When the plural noun is a non-living item, the adjective is usually a singular.
Example: هذه الأحذية جميلة (These shoes are nice)
Notice how we use هذه (This) not هؤلاء (These) to refer to the shoes, as if we were referring to a singular female object
This is not the case when talking about living people
Example : هؤلاء الفتيات جميلات (These girls are nice)
Here, we use the plural (These), and the plural adjective
I hope this helps