An AirTag does not require or require charging because Apple designed them with user replaceable CR2032 batteries. It is intended to last roughly a year before needing to be changed. The batteries are user changeable, and to switch them out, squeeze and twist on the rear panel of the AirTag to snap it out.
Apple warns AirTag owners to avoid using bitter batteries. The bitter coating used to keep children from eating coin batteries might create AirTag issues. To remove bitter coating, you can use some rubbing alcohol or acetone, but I would recommend just buying a regular brand without it.
The “CR” in a CR2032 battery, or any CR designated battery designates that its positive cathode is made of Lithium manganese dioxide. This gives it a high energy density while also allowing it to function in low-temperature environments.
When not in use, CR2032s are thought to have a 10 year lifetime. Does it indicate it has 10% of its life left, or 50% at the conclusion of that time period? I'm not sure. However, after resting for a year, one would predict that 10% of its life would have been diminished, more likely closer to 5%.