Ratio = Math .. numbers of teeth on the hub cog and the number of teeth on the chainring ..
chainring turns the hub X number of times, in proportion to their relative size.

an IGH the gears are a percentage / fraction of the direct drive gear... 1:1 is when the hub shell turns at the same rate as the cog.

you choose the 1 cog and the 1 chainring to determine the size of the 1:1 gear.