As you can see, Apple's forum software blocks some content automatically. You control the apps on the phone by installing them yourself, then blocking installation of new apps. As an alternative, set up the phone with its own iCloud ID that is linked to your Apple ID using Family Sharing. You then have the option of approving or disapproving apps that they want to install, and you can also see what they want to install. So either way you control what social media (and other) apps are on the phone.
For Safari you can limit access to "adult content", or to only specific websites. I have no direct experience with other browsers (my youngest child is 34), but there are a wide variety. You will need to explore the App Store or ask for advice from other parents.
One thing to avoid is products that allow you to monitor your child's phone remotely. All of them require hacking the phone, which leaves it open to outside miscreants. They are tempting, but the risks of using them are greater than any presumed or claimed benefit. You need to have some level of trust with your children; technology can't solve all parental problems. If you feel the level of trust is not sufficient maybe they shouldn't have a smartphone.