Learn Hardcore Russian: Вставь зарядку в розетку, пожалуйста.

Вставь is the perfective imperative of вставить. It focuses on a single completed action: do it (and it will be done).
Вставляй is the imperfective imperative of вставлять and is used more for:

repeated/ongoing actions (keep inserting / insert regularly),
process emphasis (do it slowly/while you’re doing it),
or sometimes a more casual “go ahead and do it” depending on context.

Is this sentence informal? How would I say it formally or to more than one person?

Yes. Вставь is the imperative for ты (informal singular).
Formal or plural would be: Вставьте зарядку в розетку, пожалуйста. (to вы)

What does зарядку mean here? Isn’t зарядка also “morning exercises”?

Yes, зарядка can mean morning exercises. But in everyday speech it’s also very commonly used as shorthand for a phone charger / charging cable / charging unit (i.e., зарядное устройство, colloquially also зарядник).
Context usually makes it clear: if you’re near an outlet and devices, зарядку = charger.