Is the Battery Health Management feature available for every MacBook?

This feature is available on all MacBooks with Thunderbolt 3 ports running on macOS Catalina (v10.15.5) and higher

Do I need to enable it manually?

The optimized battery charging option is enabled by default on all supported devices. However, you can toggle it any time at System Preferences – Battery.

What does "Charging on Hold (Rarely Used on Battery)" mean?

It says, your MacBook is currently connected to a power source but the battery is not being charged. The MacBook is running by drawing power directly from the AC supply. This is completely fine and good for your laptop.

What does "Battery on Hold" will be charged by 3:30 PM mean?

It says, your MacBook has monitored your charge patterns and guessed you may connect the MacBook to the power supply for very long hours. So it decided to hold the battery at 80 percent for a few hours. By the time the clock ticks at 3:30 PM, your MacBook will be charged 100 percent. This is done to reduce stress on the battery thereby improving the battery lifespan.

Do I need to worry if I see a "Charging on Hold" or "Battery on Hold" message?

No. It is completely normal and you must be happy about it. Your MacBook has monitored your charging patterns and trying to improve the battery lifespan.