Staff(meaning a collection of employees) is both the singular and plural of the noun. Staff is a collective noun so you need to think if the staff are performing an action together or separately.
The staff are meeting in the teacher’s room
The staff does all their work from home.
In reality, it depends more on where you are from. I am from Ireland and using “staff” with a singular verb sounds strange to me.
In American English, it is much more common to use a singular verb with the word “staff”.
The staff is from all different backgrounds.
In British English, it is more common to use a plural verb with the verb “staff”
The staff are from all different backgrounds.
When you use “ a member of” or “members of” then you need to conjugate staff accordingly.
A member of staff is going to help you
Members of staff are going to go on strike.
The word “staff” also means a stick and is another word for “stave” in musical notation.
These alternative meanings do use the regular plural with “staffs” being the plural for more than one stick and “staves” the more common plural in musical notation.