You may call it a subsidiary all you want, but it's still a company that's wholly incorporated in foreign soil, doing business in foreign soil.

At least in Brazil, companies that operate there must obey local laws. What happens when those laws are in contradiction with US laws, like in the example I cited? Is Brazil supposed to cave? Is Brazil supposed to keep fining Dell Brasil until it folds? Maybe prosecute Dell Brasil's directors for actively and repeatedly disregarding the law and fines?

How does that work on a global scale?

I'll say again, this is not about a US company opening a foreign subsidiary to do things in the US that are forbidden in the US. This is about a company incorporated abroad having to follow US laws while operating wholly abroad. This is a breach of sovereignty however you look at it.

It is plainly routine for a company to have to deal with multiple legal jurisdictions at a time.

Yes, sometimes this causes compliance complication. This isn't unusual, it happens frequently.

Ultimately, every government exercises the laws of their country as they see fit, using the enforcement tools they have available to them. These rules often extend outside of their borders and apply to foreign or partially-foreign entities depending on the situation. The only limits on this are the practical means of enforcing it.

Dell Brazil would have been subject to Cuba sanctions because it was controlled by the US parent company. The US has obvious jurisdiction over Dell Technologies the parent company, and the nexus to enforce it.

Nothing you are are describing is even remotely unique to the US. No country is going to let you set up a foreign subsidiary to launder goods around sanctions law. If they did, everyone would do that and nobody would ever follow sanctions.