All apps have network access by default and there’s nothing you can do about it without jailbreaking.
[1] as many pointed out: open source in iOS is a moot point as there’s no way to verify the binaries.
There’s an important distinction. On the first instance the app can’t access the outside world. On the second you will just know that it did.
[Edit]
See the author of keepassium commenting on the same issue about a month ago:
https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...
There is another option - buy an iPhone in China:
iOS: Disable WiFi (not just cellular) for specific apps without jailbreaking https://tinyapps.org/blog/202209100700_ios-disable-wifi-per-...
Have been using it for 3-4 years now. Its integration with the Apple ecosystem is second to none. I do use an online version that syncs with iCloud so I can access it anywhere (but with a Yubikey).