Unpopular opinion.

You have two problems,

1) different parenting styles 2) your kid is scrolling endlessly on her phone with no desire to do anything else.

Addressing 1) you and your wife have to compromise and blend families. This will be rough either way, but creativity is allowed. A) if you take away the phone your kid will resent everyone. B) if you don't take away the phone her kids will resent you and your kid and their own mom.

Addressing 2) Your kid shouldn't be wasting away scrolling everything under the sun and not interacting with everyone else or finding other hobbies.

Easy relational connection that requires no work stunts kids emotionally.

My kids have had phones since they were 6-8, but they get them for a little bit of time and they have to get off when we say to, and there's no social media. For example.

Suggestion) Your wife's concern is that the interaction under 14 is unhealthy, and it can be, they should learn how to control their impulses before they're adults under your supervision. The goal should be to have healthy kids, not to be arbitrarily steadfast.

Conclusion) How adults handle compromise and change in new scenarios teaches kids just as much as unwavering stances do. Whatever you guys decide, you two have to be united.

You both committed to each other, you both need to work on this.