Marvel Cinematic Universe (MCU) - How jane becomes thor

Thats a piece of mjolnir that hela broke in ragnarok
And thor 4 will feature jane becoming thor

What happened in the comics was Thor became unworthy to use the hammer.

Its worth noting that Thor is both one guy's name and also a title. So Thor Odinson became unworthy and goes by "Odinson" for the duration, while Jane Foster is worthy to use Mjolnir and thus has the title of Thor and the powers.

I really doubt the MCU is going to go into the story that involves Thor Odinson becoming unworthy because that story has some baggage to it and half-sucked. More likely, Thor will keep his axe and by some situation Jane will have the hammer or something similar.