Bill buys a large can of paint. The size of the can is measured in which unit?

Pressure is a mechanical (non-electrical and non-magnetic) measure. All mechanical measures can be derived from just three basic mechanical units. Which three we use is actually arbitrary. This means that using three we can produce or define all other mechanical measures by multiplying or dividing some combination of the chosen three.

Most measuring systems use time, length, and mass as the basic three in which case pressure would be mass / (time * time * length * length). Compare that with kilogram per (meter*meter * second*second) which is also called a Pascal.

Another measuring system might use time, length, and force as the three basic measures. Here mass is omitted in place of force. Nothing is lost because mass and force can be written in terms of the other with the help of time and length. In this case pressure would be force / (length*length). Compare this with Newton per meter*meter which also defines a Pascal.

How pressure is measured in practice is another good question. The standard method is to use some device that employs a medium to compare the unknown pressure to some known familiar pressure. Mercury tubes illustrate this nicely and are still used in laboratories. They work because mercury is a very dense fluid. A lighter fluid can push upon and move a column of mercury but cannot pass through it.

Measuring the movement of mercury when a known pressure is applied to one end reveals the pressure at the other end. The persistence of millimeters of mercury (mmHg) as a unit for reporting pressure is testament to this method. Other pressure gages work by employing something like springs or strain sensors calibrated to mimic the readings that a mercury tube would report.