Is "ser" always used to express something permanent and "estar" for something temporary?

This is a common misconception when learning to distinguish ser and estar. There are many rules and exceptions regarding this. Here's an example: "Mi abuela es mayor" vs "Mi abuela está mayor". Some adjectives in Spanish indicate reality or appearance depending on which verb they're followed by. By reality we understand what is inherent in a person (eg. my grandma 80 years old, so I could say "Mi abuela es mayor") and by appearance we understand what something/someone looks like but it’s not (eg. my grandma puts on lots of rejuvenating lotion, does a lot of sport and takes care of herself so she looks like she's 60 years old: "Mi abuela es mayor" (reality: she is old) but "mi abuela está joven" (appearance: she looks young).