what if by accident it fell in the swimming pool

If the device is off, let it sit for several days. Do not put it in rice! Do not put it in the microwave! Then, power it on.

If it still works, back it up ASAP, then get it inspected at a licensed location if you wish (Apple - Find Locations).

However, if it doesn't work anymore, then you'll have to get a new phone. If you have a backup of your iPhone (to iCloud or a computer), then you're fine- import the data with the backup as shown here (select the applicable method) - Transfer data from your previous iOS or iPadOS device to your new iPhone, iPad, or iPod touch - Apple Support

If you didn't have a backup, there's not much you can do about it.

Above is correct, save that AppleCare+ does cover liquid damage with payment of a service fee. See section 3.2:

https://www.apple.com/legal/sales-support/applecare/applecareplus/2205/220606_applecareplus_us.pdf

Backups are one of the very few ways to indicate that data has value. No backups means the data has no value. We all learn this lesson, and all too often through data loss.