To Get Both Mew and Jirachi you will need to have played some of the previous pokemon games. To get Mew you will need to have a save for one of the Let’s Go games. But for Jirachi, you will need to have a save for either Sword or Shield.
If you have gotten a save for one or both of these games, you will need to reach Floaroma Town. In Floaroma Town you will find an elderly couple in a flower field on the bottom left side of the town. Talk to each of them to get the Pokemon.
The Elderly man on the left will reward you with Jirachi and the Elderly lady on the right will gift you a Mew.