It’s actually both.
On the one hand, “any wine” can be used, so long as it meets the Church’s requirements.
On the other hand, some wineries specifically bottle and label sacramental wine to make things easier. There are only a very small number who do this (maybe 5 nationally); with a wide selection of wines.
Some bishops require that priests use wine that is specifically labeled as altar wine. Others don’t.
Sometimes, it’s not cost-effective for small wineries to make special “altar wine” labels because they just don’t sell enough. I know of a few where they cooperate with the local bishop. They sell their regular off-the-rack wines as altar wines, but they have a list of wines that the bishop has approved. When the purchaser says “I’m using this for altar wine” the clerk takes out a sheet of labels which read something like “approved for sacramental use by the bishop of…” and adds them to the bottles. Nice little system.
Some small wineries might label one or two varieties as “altar wine” without actually having a full selection of labeled altar wines.
The wine itself is the same; only the labels are different.
Some priests know that wine they buy in the store is exactly the same wine (produced by the same winery) that gets sold as altar wine. There’s no problem doing it that way, as long as the priest is certain that what he’s choosing is a valid choice.