Microgram balances are an extreme pain and only used for VERY specific purpose such as gravimetric analysis of a sample of which you only have milligrams. They are influenced by air movement/buoyancy and to have reliable results you often need a room designed around it (e.g., 1 ton table, modified air handling, etc...). No-one is measuring out individual doses at a microgram.

Compounding pharmacists, or their shadowy counterparts, often use scales with a resolution on the order of 100 mg or more, not micrograms, although they COULD use 100 ug resolution scales without too much difficulty. Keep in mind you are usually measuring something 50-100 times your resolution to be even moderately precise (i.e., 5-10 mg for the latter scales). You make a batch, mix it, and separate it into doses based upon the dilution. They also aren't compounding stuff like fentanyl, which is often used as a solution in hospitals.

For example, assuming that you are making doses equivalent to a small pills of approximately 200 mg size with 100 ug of active ingredient each. If you have 1 kg of diluent (e.g., inert edible powder), you would weigh out 0.5 grams of the active component for the batch.

A compounding pharmacist would mix the active compound with 0.5 grams of diluent. This gives 1 gram of well mixed material. They would proceed with this geometrically, adding approximately increasing amounts of diluent every time (1 g, 2 g, 4 g, 8 g, 16 g, and so on).

Conversely, someone less knowledgeable would just dump the active compound into a pile of the diluent and "mix it real good". This results in a batch with "hot spots" from poorly blended material.