Dwarf galaxies with stellar masses of 10⁹ Msun can be explored at high and low redshifts and they give a glimpse of the different conditions of galaxy formation at different epochs. Using a large sample of about 300 zoom-in cosmological hydrodynamical simulations of galaxy formation I will briefly describe the formation of dwarfs at this mass scale at 3 different epochs: cosmic dawn, cosmic noon, and today. I will describe the FirstLight simulations of first galaxies at redshifts 5-15. These first dwarfs have extremely high star formation efficiencies due to high gas fractions and high gas accretion rates. At cosmic noon, z = 2, galaxy formation is a violent process. The VELA simulations have generated a set of dispersion-dominated dwarfs that show an elongated morphology due to their prolate dark-matter halos. Between z =1-0, the AGORA simulation shows the formation of a low-mass disc due to slow gas accretion.