Reduced Relative Clause

I have found this sentence in some of my books and I come across with this sentences below. What I am not clear is the first and second bolded part.

“That carcinogenic substances are contained in many common household items is well known”

First bolded

• I think wouldn’t it be correct if it was only “contained” which is a short form from (which are contained)

Second bolded

• Not sure, why the verb “is” was used here since the subject is plural.

Sorry if the title is not related