Abstract: Flagella/cilia-driven planktonic microorganisms represent typical microscale self-propelled systems, whose motility is jointly regulated by low-Reynolds-number hydrodynamics, propulsion structures, and collective interactions. From the perspective of experimental fluid dynamics, this review follows two main lines: environmental complexity and organizational scale. It summarizes the propulsion, orientation, reorientation, and fluid responses of single swimmers in quiescent, confined, and shear/external flow environments, and further discusses aggregation, vortical motion, pattern formation, and transport evolution at the collective scale. Existing studies show that flagella and cilia serve not only as microscale propulsion structures, but also as key interfaces connecting biological behavior with hydrodynamic regulation. Hydrodynamically mediated interactions play an important role in cross-scale self-organization. Finally, future directions are discussed in terms of experimental characterization, complex environmental coupling, and bioinspired applications.