Are smart chimes brand-locked to specific doorbells?

Generally yes. Ring chimes pair with Ring devices, the myQ Chime works with myQ video doorbells, locks and garage keypads, and bundled chimes (Reolink, Aqara, eufy) are designed for their own ecosystems. Before buying a standalone chime, confirm it lists your exact doorbell model as compatible, this is the most common source of disappointment in customer reviews.