For any Mac made in the last 10–12 years, you typically do not buy a physical "piece" of software.
: Apple generally provides full security protection for the last three major macOS releases. To help you find the right "piece," could you clarify: buy osx
: Specialized retailers like iFixit and OWC (MacSales) offer DIY kits for internal upgrades, including batteries (approx. $80–$135 ) and SSDs. For any Mac made in the last 10–12