My understanding is that the only legally required information that the US Census can force you to respond to is the number of people residing in a household. They typically threaten that you have to answer any question they ask but my understanding is that they can ask but they actually can’t force. I’m not an attorney though.
No, you are legally required to respond to the Decennial Census, American Community Survey, and the Economic (business) Census.
I don't understand why people are so paranoid about the Census. The Decennial Census is used to count the number of people. That information is used for redistricting and apportionment. Federal funding is based on population. That's money for schools, road improvement, infrastructure, disaster aid, and many other things.
The American Community Survey expands on the Decennial Census. Through the information gathered in it we can correlate income levels and demographics. What kind of jobs (job sectors) people have, where people are commuting to and from to get to work, educational attainment levels, and other trends. The types of new vehicles people have been purchasing over the last decade has shifted away from cars to SUVs and trucks. I can only speculate that the ACS is gathering data to get a better view of the trends. The data is then released and is public, but depersonalized. Various government agencies can use the data for things like future planning. If the data shows more people and more cars, that means more funding for roads. Businesses and the general public also have access to the data. Just as an example, if you develop shopping centers and you are building a new shopping center where there is a disproportionately large number of SUV/truck owners you might consider making the parking stalls large enough to accommodate something larger than a go cart. If you are building a shopping center where there are a lot of senior citizens, maybe you add more disabled parking stalls. Or, charging stations for areas where there are a lot of electric vehicles.