How to make an introvert happy
Let them be quiet or alone for as long as they need. Yes, they actually need that to function correctly.
Make the first step. It is likely easier for you than for them, but in the end you will both gain from it.
Know that they typically prefer quality over quantity. Less relationships, but better. Less crowded spaces, but more intimate. Less talking, but more listening.
Do not try to change them. Their personality is on a spectrum, just like everyone else’s. It will evolve with time, but it is not an on/off switch.
Cancel plans you had with them. Chances are they will secretly be thanking you, and enjoy some quality time with themselves instead.