I couldnt get a phone until i started highschool (i had very limited access to a tablet at home). This resulted in me being unable to participate in any of the group chats that my peers were using, and missing the necessary context to understand a significant amount of topics they discussed even in person. Up to the point smartphones started to spread in my class i was strongly involved in the community, and i would say i had sufficient social skills. After that i started to get socially isolated, and this i would say severely affected my social development for many years.

Nowadays im happy that i spent most of my free time reading and learning extracurricular topics while many other were binging youtube, but its only because in the last couple of years i successfully started to develop my social side and engage more with others, while keeping the benefits of being left alone with my thoughts for extended periods. However i wouldnt have been able to do this on my own (i convinced myself that my isolation is a good thing, and as a coping mechanism i looked down on others socialising, smalltalk, etc.), and was very lucky with a couple of people that got me out of this isolation. That said i still have to undo a lot of damage on this area.

I dont know how a parent could balance these things, but i would assume that the most important thing is to help the kid find hobbies that engage them, so that scrolling endlessly is not that enticing, while giving them time on their phone to nurture their relationships online (this could be restricted with scheduling wifi access on the router, etc), and of course educating them on the potential harms of the internet.

Also i dont really have a solution to this, but i noticed on myself that when i had restricted access to something (for example the wifi turned off at 8pm) that meant the restricted activities value went way up in my head and i maxed out on it, often even though i would have enjoyed doing something else more.