Account Management
You can manage accounts at Settings > Passwords & Accounts. This is where you can add exchange and gmail accounts. For each account, you can also select what data to import. Data can include calendar items, mail items, and contacts. The data will show up in the native apple apps. Any time accounts are added or removed, be sure to adjust the default accounts so that new contacts and new calendar items are being added to the correct account.
Default Accounts
You can set default accounts for calendar items and new contacts under Settings > Mail, Contacts or Calendar. Under each, look for Default Account or Default Calendar. When new items are created, they will be added to the account selected.
Choosing Calendars
Within the calendar app, you can select which calendar to display by selecting Calendars at the bottom.
Also, when selecting a specific event, there is an option, Calendar, which lets you move events from one calendar to another.
Configure WiFi
WiFi can be configured from Settings > Wi-Fi. To reset a connection, click the information button 
. At the top, there should be an option to Forget This Network.
Updates
To update the OS, Settings > General > Software Update.
To update apps, App Store > Updates. As of iOS 13, you can't see apps available to update since it is done automatically.
Backups
To configure what gets backed up to iCloud, go to Settings. Tap the top menu item that says your name. From there, go to iCloud. Within that list, select what items to back up. Warning: Only 5 GB is available. Most people have pictures more than this amount, and iPhone full backups take up most of that space. More than likely, you will have to use other apps (like Google Photos) to backup photos, and save the iCloud storage for specific apps such as Messages, Health, Keychain, and any apps at the bottom. And contacts / calendar items should be stored in a gmail or exchange account. Full iPhone backups are not necessary.