You get access to Mega Evolution pretty late in the Let's Go story. The Pokémon trainer Blue is seen throughout the game, but he appears once again after you win your 7th Gym Badge on Cinnabar ...

How to get two mythical Pokémon in BDSP. Now it’s time to get Phione. Once you’ve hatched your Manaphy egg, head to the Pokémon day-care and leave the Pokémon there along with a Ditto.