Why would a senior engineer do a system design? Isn't that a role of architect? For me that questions are in line with the "how much tennis balls would fit in

Why would a senior engineer do a system design? Isn't that a role of architect? For me that questions are in line with the "how much tennis balls would fit in an airlplane?" - fun, but are orthogonal to the SWE roles. How often SWE does a system design? Maybe in small startups.