You cannot board the light rail (nor BRT) and present cash. You have to buy an Orca card first, which are available in various locations, but chiefly, none of those locations are inside the light rail or BRT.
Ticket vending machines, which sell both ORCA cards and fare tickets, are available at every light rail station.
For BRT, you can pay cash at the time of boarding.
You can pay with cash at the ticket machines at all of the link stations, not just Westlake/large stations.
You can buy an orca card for $5 and then add money on it at multiple places.