> I assume the big brands get part of the way there by mixing old and new wine, though.

I don't think they can - wine is usually sold by vintage year. "Origin" is usually weaker (e.g., 50% of grapes must be from the declared origin) and my understanding is that is where you can "adjust" year-to-year.

Some of the cheap brands (Barefoot for one) don’t have a vintage. So I think they can do a solera-type system for their wines.

Not mixing between years, but you can mix between different vineyards of the same year. As long as the wine comes from the same region you can keep the branding. There are plenty of contract vineyards that sell to different wineries to enable these mixes.