Microsoldering essentially means soldering with extremely small components, often using a microscope.
Microsoldere are people who do microsoldering and have much broad knowledge in term of electronics and how different circuits work on different devices. The most important expertise of a microsolderer is to diagnose a device correctly and find its fault. This can’t be easy if you lack basic understanding of how electronic components works or how to read schematic and having years of experience getting familiar with how different models and brand’s circuit boards are designed.
You can also find that people refer to microsolderers as level 3 technician. In short, level 1 is mostly software related. Level 2 requires opening the device and perform troubleshooting to isolate the fault.