Extremal black holes have long been regarded as unphysical objects that cannot arise via gravitational collapse in physically plausible scenarios. In the asymptotically flat setting, this paradigm was overturned by Kehle and Unger, who used characteristic gluing to construct regular, spherically symmetric solutions to the Einstein?Maxwell-charged scalar field system that collapse to form an exactly extremal Reissner?Nordström black hole in finite time. In this talk, we explore how these techniques can be extended to include a positive cosmological constant, yielding solutions to the Einstein-scalar field system in which an exactly extremal Schwarzschild?de Sitter black hole is formed. We also discuss the dynamical formation of the related extremal Schwarzschild?de Sitter white hole and Nariai spacetimes.