A client pays their stylist 20% and assumes that covers everyone who touched their hair that visit — including the assistant who spent ten minutes washing and conditioning it before the cut even started. It usually does not.
Who actually gets tipped
The main stylist or barber: 15-20% of the service cost in the US, roughly 10% in the UK and much of Europe, optional rather than expected in most of Asia.
A separate shampoo assistant, if one washed your hair before the stylist began: a small separate amount, often $3-5 in the US, handed to them directly.
A colourist, if different from your cutting stylist, and you had a colour service: typically tipped separately on their portion of the bill.
The self-employed exception
Many hairdressers and barbers, especially in independent salons and barbershops, rent their own chair and operate essentially as a one-person business within the salon. For these stylists, cash handed directly — rather than added to a card payment that might be processed through the salon's own system — is often genuinely preferred and arrives with them faster.
Outside the US: in the UK, Ireland and much of continental Europe, around 10% or simply rounding up is the norm. In Japan and South Korea, as with restaurants, tipping a stylist is not customary at all and may be politely declined.