They measure them in light-years.
Yes.
Usually such distances are measured either in light-years, or in parsecs.
Stellar distances, like the distances of stars and galaxies from Earth
The same as linear distances: kilometres, metres, centimetres, millimetres, etc.
No, a second is a unit of time, not a unit of distance. It is used to measure time intervals, not distances. Distances are typically measured in units such as meters, kilometers, or miles.
light years
You can use the kilometer to measure small distances, but it is inconvenient to do so. A kilometer is a unit of measurement for a large distance. It is more accurate and practical to measure small distances using smaller units like the meter or centimeter.
For small distances, e.g. Earth to Moon, scientists use miles of kilometers. For larger distances, e.g. the orbit diameter of Jupiter, they use the "AU" or Astronomical Unit, which is the distance from the Sun to Earth. For enormous distances, astronomers use the lightyear, which, although it sounds like a time unit, is truly a distance unit. A lightyear is the distance light travels in one year, or 5.87849981 × 1012 miles. Another unit for large distances is the Parsec, which is 3.26 lightyears.
They do not use units: they use the fact that stars are not galaxies. For example, you don't use units to measure the difference between children and countries.
In the context of measuring angles or distances, nanometers and radians are not directly related units. Nanometers are used to measure very small distances, while radians are used to measure angles. They are different units that serve different purposes in the field of measurement.
Light year or astronomical units
There are MANY units used to measure distances both in Metric and in English types. We use Feet and Yards to measure distances in the US. The academic field often uses metric units like meters and kilometers to measure distances. For larger or massive distances there are the units of miles (English) and Kilomiters (Metric) and then in space we have the Astonomical Unit (Distance from earth to Sun) and the Light Year for the biggest distances. And there are the smaller measures of inches and millimeters.