row of carriages, generation, class, series a kind, denotes the plural
軰 centers on a connected line or group: a row of carriages, a generation, a class, or a series. The plural sense extends naturally from the idea of a collective set.
軰 is a variant form of 輩. The standard form 輩 combines 非 (likely phonetic) with 車 (carriage), suggesting a row or group of carriages. The variant 軰 replaces 非 with 北, which may have arisen as a simplification or visual corruption.
The top 北 looks like two people back-to-back, and the bottom 車 is a carriage. Imagine a long row of carriages lined up like a generation of people standing together.
For ハイ, picture a long row of carriages stretching into the distance, and you wave and say 'Hi!' to each one: Hi -> ハイ.