If you "do business" in another state — employees, office, regular sales, real estate — you must register there as a foreign entity. Skipping it = fines, loss of good standing, inability to sue in that state.
"Just shipping a product" usually doesn't trigger this. "Hiring a remote employee in California" definitely does.
Next action
Turn the lesson into work.
Pick one concrete next step now, while the context is still fresh.