plugin charging always doesn't make your battery life decrease.
It depends on the amount of time you use and also using multiple browsers/high loaded tasks at a time.
To enhance your MacBook Air's battery life, consider adjusting system settings such as lowering screen brightness, enabling energy-saving modes, and reducing display sleep time.
Manage applications by closing unused apps and minimizing background activity.
Disable unnecessary features like Bluetooth and Wi-Fi when not in use.
Keep your software updated to ensure optimal performance.
Maintain battery health by avoiding deep discharges and calibrating the battery periodically.
Additionally, avoid exposing your MacBook Air to extreme temperatures to preserve battery performance