High-quality superconducting single crystals and thin films play an important role in the basic research and application of high-Tc superconductivity. In these two aspects, iron-based superconductors feature the merit of rich physical phenomena and high superconducting critical parameters (including the transition temperature Tc, the upper critical field Hc2 and the critical current density Jc). By developing ion-exchange and ion-de-intercalation method, we successfully synthesize a series of high-quality and sizable (Li,Fe)OHFeSe and FeSe single crystal samples. We observe Ising spin nematicity (below Tsn), and the universal linear relationship between Tc and Tsn in FeSe single crystals, indicating that the superconductivity is closely related to the spin nematicity driven by stripe antiferromagnetic spin fluctuations. In (Li,Fe)OHFeSe single crystals, we observe the coexistence of an AFM state (below Tafm~125 K) together with the SC state. We explain the coexistence by electronic phase separation, similar to that in high-Tc cuprates and iron arsenides, and establish a complete phase diagram for (Li,Fe)OHFeSe system. Here, we also make a brief introduction about our latest progress in growing a high-quality single-crystalline superconducting film of (Li,Fe)OHFeSe. The film is prepared by a hydrothermal epitaxial method. The high crystalline quality of the film is demonstrated by x-ray diffraction results, showing a single (001) orientation with a small crystal mosaic of 0.22 in terms of the full width at half maximum of the rocking curve, as well as an excellent in-plane orientation by the -scan of (101) plane. Its bulk superconducting transition temperature Tc of 42.4 K is characterized by both zero electrical resistance and diamagnetization measurements. Based on systematic magnetoresistance measurements, the values of upper critical field Hc2 are estimated at 79.5 T and 443 T for the magnetic field perpendicular and parallel to the ab plane, respectively. Moreover, a large critical current density Jc of a value over 0.5 MA/cm2 is achieved at~20 K. Such a (Li,Fe)OHFeSe film is not only important for the fundamental research for understanding the high-Tc mechanism, but also promises the high-Tc superconductivity applications, especially in high-performance electronic devices and large scientific facilities such as superconducting accelerator.