What is done when a house's sewer line is below the level of the main sewer
line? I didn't think those lines were pressurized, so how does a house's
sewage system work in those conditions?

For instance in my neighborhood half the houses are about 20 feet below my
street. In this instace would the city install a pumping unit for those
houses or would they route the sewer line towards those houses and out? I
guess it depends on the situation, but still, if the waste treatment plant
is above your house (vertically that is) does that mean your sewage is
pumped to that plant or is sewage all gravity fed?

Always wondered about that, I never hear pumps running under the street and
I didn't think lines were pressurized anyway.