No, not by default. That would be impossible, since iCloud Backup first requires an iCloud Account, and then requires enough iCloud storage space to backup your phone. You get 5GB for free, which is not enough for a lot of people.
Mine doesn’t. It’s ostensibly expected that a backup is going to send your data somewhere. I don’t use iCloud backups for that reason. Also, some iCloud backup data is e2ee. The document it here: https://support.apple.com/en-us/HT202303
Data types that are protected by end-to-end encryption—such as your Keychain, Messages, Screen Time, and Health data—are not accessible via iCloud Data Recovery Service. Your device passcodes, which only you know, are required to decrypt and access them. Only you can access this information, and only on devices where you're signed in to iCloud.
Amusingly, your comment is the actual lie. iCloud backups are an opt in cloud backup solution. They are not enabled by default and myself (and many others) have never used them.
I've set up four iOS devices from scratch (new numbers, new Apple IDs, new everything) in the last two weeks and iCloud Backup is indeed on by default.
https://www.reuters.com/article/us-apple-fbi-icloud-exclusiv...
https://www.theverge.com/2020/1/21/21075033/apple-icloud-end...
The iPhone uploads very nearly the entire contents of the device every night by default to Apple, in a format Apple can easily read.