Going off of what Mike listed, here's what I would list Dennis since most are the same as the original:
How To Get The Mythical Pokemon Mew:
If you have a saved game file for Pokemon Let's Go Pikachu/Eevee on your Nintendo Switch, you can obtain a gift Mew from the lady to the southwest of Floraroma Town.
How To Get The Mythical Pokemon Jirachi:
If you have a saved game file for Pokemon Sword/Shield on your Nintendo Switch, you can obtain a gift Jirachi from the same lady to the southwest of Floraroma Town.
How To Get The Mythical Pokemon Manaphy:
You can obtain a free Pokemon Egg as a Mystery Gift before Feburary 21st, 2021, and it will hatch into Manaphy. This Manaphy can be bred at the Pokemon Daycare to obtain the Legendary Pokemon Phione.
How To Obtain All 1st, 2nd & 3rd Generation Legendary Pokemon:
After defeating the main game, you will earn access to the Battle Area. In the original version of the games, this is where Pal Park was located. However, it has now become Ramanas Park, which can be accessed after obtaining the National Pokedex. Here, you can find all the Legendary Pokemon from the 1st (Kanto Region), 2nd (Johto Region) and 3rd (Hoenn Region). However, in order to do so, you must obtain a Legendary Pokemon's Slate from one of the sellers in Grand Underground. Below are what Pokemon you find with what slates at Ramanas Park...
Kanto Slate (Exclusive To Shining Pearl):
Johto Slate (Exclusive To Brilliant Diamond):
Rainbow Slate (Exclusive To Brilliant Diamond):
Squall Slate (Exclusive To Shining Pearl):
How To Change Giratina's Form:
In order to change the Legendary Pokemon Giratina into its Origin Form that was introduced in Pokemon Platinum, you must obtain the Griseous Orb. Unlike previous games, this is a little tricky to do. First off, you must complete the main game and obtain the National Pokedex so that you can gain access to Ramanas Park. From there, you must buy the Distortion Slate from an seller in the Grand Underground. However, you won't just obtain the Griseous Orb by inserting this slate at Ramanas Park. Instead, this will gain you access to the Distortion Room. Here, you can find and battle Shadow Giratina. You must defeat it in order to obtain the Griseous Orb.
Benefits From Partner Pokemon:
Like a few previous Pokemon Games, you can choose to have a Pokemon follow you outside of its Poke Ball. This will start earning your Pokemon extra benefits while in battle, and most older players are dubbing this as Easy Mode. These benefits include your Pokemon automatically being healed of status conditions in battle and hanging on from an attack with one hit point. The more you have your Pokemon follow you outside of their Poke Ball, the higher the odds of these benefits activating.