If your ERP is one of the bigger names, NetSuite, SAP Business One, Microsoft Dynamics, Acumatica, there's a decent chance a packaged connector extension exists for it, sold either through the Magento Marketplace or directly by a third-party vendor. These handle the basics: new orders flow to the ERP, maybe inventory flows back, usually on a schedule rather than instantly.
Where no packaged connector exists, or where the one that does exist doesn't fit, the fallback is manual CSV export and import. Someone pulls an order report out of Magento, reshapes it to match what the ERP expects, and uploads it, often a few times a week, sometimes daily if volume demands it.
A step up from both is middleware, tools like Celigo, Boomi, or Jitterbit that sit between the two systems with more flexible field mapping than a rigid packaged connector offers. It's a real improvement, but it's still template-based integration work, just paid for through a subscription instead of a one-time build.