How Mir advanced the space station technology during its remarkable 15 year long history in space?
It was the first modular station - multiple large modules, each one closer to habitation module rather than to a transport spaceship of the time. Mir also featured module re-docking. The base module had the docking unit with 5 docking ports, and all modules were first docked to the axial port - and then with the help of a specialized manipulator those modules were moved to radial ports. The station was working and habitable before docking, after first docking, after redocking and during those operations.
Mir had pretty long expeditions - and the continuous spaceflight of Valeri Polyakov is still the longest one in 2026. The expeditions were overlapping, so the station didn't need to be prepared for autonomous flight and switched back into manned mode each time a new crew arrived. For more than 3600 days Mir flew continuously crewed, providing the humanity's permanent outpost in space.
Mir modules were planned to be dedicated to different scientific areas, and seriously advanced space science, paving the way to ISS. But it's not the only area where Mir was a precursor...