The following article outlines how to lock down a teenagers phone and the considerations in doing so.
Honestly, most parents have no idea the harm that unrestricted access to the internet is having on their kids (pornography being only one facet), let alone what to do about it. In many ways, the internet is like a chainsaw: incredibly useful if used appropriately, but highly dangerous otherwise.
We used to market cigarettes to children and look back on those who allowed it to happen with self-righteous condescension. However, we now portray parents who want to limit what their kids have access to online as insecure, prudish helicopter parents. If you want a very rude shock into what your kids are doing online, watch the series "Mirror, Mirror" by Todd Sampson.
Google does have some very good parental controls built in at the system level (i.e., not app lock overlays). The app that manages everything that you need to install on the parent device is known as Family Link. It enables you to limit what apps are used, for how long, and even times for the phone to shut down, which can help manage things like dinner time and sleeping.
One issue with Family Link is that once the child reaches 13 (or the applicable age for the country the child is in), they can opt out of the parental controls. If they do so, you will be notified, and their phone will be locked for 24 hours. If you don’t want them to be able to unlock the phone at all, the easiest solution is to make a dummy account and set its age to something like 9, which would give you another 4 years to monitor the device. I would recommend against this if possible, as you risk driving the behaviour underground. It is much better if you and your child can both agree on the systems that you are putting in, that you have their best interests at heart, and also that there is room for negotiation.
Once the child account is set up, agree with your teenager what apps they need for learning and general use, and lock everything else (particularly Chrome, YouTube, Google Search, and all social media). You will also need to disable the ability to install new apps via Google Play.
Apple has a similar set of features built into their operating system, more information about that can be found here.
By far the biggest weakness at the moment with Family Link is that there is currently no way to disable creating a mobile hotspot with the phone. That means if your child has access to a laptop or some other computer, they can create a hotspot with their phone and then use the laptop to have unrestricted access to the internet. This is a big problem, as it enables your child to circumvent the restrictions you have in place with your home router.
It is possible to use an applock to stop your child from accessing the settings app to turn on the hotspot there; however, if the phone has a hotspot button in the drop-down menu, they will still be able to access it there. The only fail-safe way I know to prevent hotspotting is to modify the hostapd file on the phone; however, that is a non-trivial task as it requires root access to set up.
A lesser weakness of the setup is that some apps have their own in-built browser that kids can use to browse once the app in question is installed. You should do a check on each app that is installed to make sure it cannot browse the internet.
The end goal is to teach your kids that the internet is a very useful tool and should be viewed as such. It needs to be treated with the same respect that other powerful tools are (chainsaws, angle grinders, reciprocating saws, etc.). It is not the best place to spend your leisure time, and definitely not where you should be going to derive your self-worth.
Unrestricted access to browse the internet should not be enabled on mobile devices. Ideally, you would have a desktop computer in a very public space in the home that children can use if they need to browse the internet. Most routers come with reasonable (but not perfect) parental controls and filters you can use to protect your child when they are using the desktop.
If you want to let your kids play computer games, consider buying a dedicated console like a PlayStation or Xbox. These get plugged into a TV, making it easier to monitor what is going on, and you also have more control over what games get played. In addition, many console games are very social (dancing, karaoke, etc.), so it is much easier to have groups of kids play together to keep each other accountable. Surely that is a better alternative to having your child play games on their own until all hours of the night, chatting with strangers around the world, all from the privacy of their own bedroom.
Finally, be mindful of what you are modelling for your children. It is difficult to regulate what your kids do with their phones if they see you on them all the time. One of the most poignant comments I have ever heard from a child regarding technology use is:
"I wish my dad would play with me more instead of being on his phone so much."