Amazon is the most native of the three: BigCommerce's own Channel Manager lets a store list products and sync orders directly against Amazon, eBay, Walmart, and a handful of other marketplaces, built and maintained by BigCommerce itself rather than a third party.
HubSpot has an official integration app listed in both BigCommerce's and HubSpot's own marketplaces, syncing contacts, orders, and basic customer activity between the two. It covers the common case well: a BigCommerce order showing up as a deal or contact activity in HubSpot without custom code.
NetSuite is different. There's no single first-party connector BigCommerce ships the way it does for Amazon; instead the marketplace has a handful of third-party connector apps built by integration specialists, typically syncing at the order and customer level. That's a real, working option, but it's a connector someone else maintains, with its own feature scope and its own limits on what it actually syncs.