Défaut browser

You have an iPad4 with iOS10 installed. This version of iOS does not support setting of a default browser. While you may be able to install and use alternative browser Apps, Safari will always remain the default.

Your iPad4 cannot be updated to a later version of iOS. Update support for iPad4 ended September 2017, the final iOS releases for this model were iOS 10.3.3 (WiFi Only models) and iOS 10.3.4 (WiFi & Cellular). iPad4 cannot be updated to any later major version of iOS as it lacks the minimum hardware requirements needed to run more recent versions.

If you need to use Apps or features that are not supported by your iPad, you’ll need to replace the iPad with a much newer model. The current entry level iPad9 supports iPadOS 15 - offers access to all current software features - and will massively outperform your obsolete iPad4.

You need at least iPadOS 14 to get that option.