Learn Hardcore Norwegian: I hverdagen kobler jeg fra ved å gå en tur uten mobil. - In everyday life I disconnect by going for a walk without my phone.

Because Norwegian uses V2 word order in main clauses: the finite verb is in position 2.
When you start with an adverbial like I hverdagen (In everyday life), the verb stays second, so the subject moves after the verb:

Jeg kobler fra i hverdagen. (neutral order)
I hverdagen kobler jeg fra ... (fronted adverbial → kobler must be 2nd)

What does kobler ... fra mean, and why is it split?

å koble fra is a common phrasal/separable verb meaning to disconnect / switch off / unplug (often mentally: “to unwind”).
In main clauses, the verb is conjugated and the particle often comes later:

Infinitive: å koble fra
Present: jeg kobler fra
Past: jeg koblet fra

It’s “split” because fra functions like a particle, similar to English switch off.

Could I also say Jeg kobler fra i hverdagen? Is there a difference?

Yes, that’s fully correct. The difference is mainly emphasis:

I hverdagen kobler jeg fra ... emphasizes when (in daily life).
Jeg kobler fra i hverdagen ... is more neutral and starts with the subject.

Why is it I hverdagen and not i hverdag or i hverdager?

hverdag = a weekday / an everyday (countable)
hverdagen (definite form) is often used more like “everyday life” as a concept.

So I hverdagen usually means in everyday life / in day-to-day life, not “on a weekday.”

What grammar is ved å gå? Why not just å gå?

ved å + infinitive is a very common way to say “by doing” / “through doing.”
So kobler jeg fra ved å gå ... = I disconnect by going ...

You can also express this idea other ways, but ved å is one of the most standard.

Why is there an å before gå?

Because after ved in this construction, Norwegian uses an infinitive phrase: ved å + infinitive.
So:

ved å gå = by going
ved å lese = by reading
ved å sove = by sleeping

What does gå en tur mean exactly? Why not just gå?

å gå en tur is an idiomatic expression meaning to go for a walk / take a walk (often a casual, pleasant walk).
Just å gå usually means simply to walk (as a mode of movement), not necessarily “go for a walk” as an activity.

Why is it uten mobil and not uten mobilen?

Both can work, but they feel slightly different:

uten mobil = without a phone (often meaning without having/using one at all, more general)
uten mobilen = without the phone (more specific: my/the phone we’re talking about)

In this context, uten mobil naturally implies “without my phone” as a general habit.

Can uten mobil also mean “without cellphone service”?

In some contexts, yes, but here it most naturally means without (a) mobile phone.
If you meant “without cell service,” you’d usually clarify, e.g. uten mobildekning (without mobile coverage) or uten dekning (without coverage).

Is kobler jeg fra something you’d say about relaxing, or is it more about devices?

Both are possible. Literally it can be about disconnecting/unplugging devices, but very commonly it’s used figuratively for unwinding / switching off mentally:

Jeg kobler av / Jeg kobler fra = I unwind / I disconnect

(koble av is also very common, often even more “relaxation”-focused, while koble fra can sound a bit more like disconnecting from something.)