String variables in Figma currently work as full replacements — a text layer can be bound to one string variable, and its entire content becomes that variable's value. There's no way to embed a variable within a longer static string, like "Welcome back, [$username]" or "Order #[$orderNumber] confirmed".
Supporting inline variable interpolation in text layers — similar to how template literals work in code — would make it far more practical to prototype dynamic content and reduce the need to split text into multiple layers just to inject a variable value.