Open up the main menu by pressing esc on pc, or tap top left icon if you are on mobile. Then go to "Party Setup".
In the party setup you can change individual party members, or tap the "Quick Setup" button at the bottom to quickly change members around like the pic below.
You can have multiple "party setups" by scrolling to the side on the party setup menu. If you want to change to the other party in your setup, you will have to tap "deploy" button at the bottom right.
Having a certain element type in your party gives certain bonuses. You can view this by tapping the "Elemental Resonance" button at the top. Having 4 unique elements gives All Element Resistance +15% bonus, and having 2 of the same gives different bonuses.
You can cook dishes in the game once you have the needed recipe and ingredients. There are a lot of places in the world where you can cook as long as there's a campfire, otherwise you can just cook at the town next to the merchant that sells food ingredients.
Once you select the dish that you want to make, you can also change the character that will do the cooking. There are certain characters that will give some kind of bonus when you do this, like Kaeya.
There are certain ingredients that you will have to "Process" and this takes real time in order to do. In the Cook screen, simply scroll to the right, or tap the process tab at the top to go to the process screen.
There are some quests where you have to be at a certain time of the day in order to progress. You don't have to wait that long in real time in order to progress though, there is a "wait" feature in the game where you can fast forward in-game time to the desired time. Tap the clock icon just below the mail icon at the very left when you open up the main menu.
Adventurer Rank rewards
Whenever you level up your Adventurer Rank, you can get some free rewards simply by talking to the guild npc in Mondstadt near the gate.