Shower instead of bathtub, what to consider?

I want to renovate this 80s bathroom.
Mostly tile on tile and a toilet with an integrated tank.
Above all, the bathtub is to be replaced by a walk-in shower. For this, the faucet also needs to be set higher. What is the thing in the red circle? It looks like a vent or an overflow. I don’t know this system…

What was new to me and has proven worthwhile for us is a walk-in shower. The advantage is that you can simply step in without having to consider a step and - less often, but no less useful - that any water, whether from a burst pipe, a broken washing machine, cleaning, or anything else, always drains here first.

The floor-level shower initially depends on the on-site drain. Because a drain that is too high prevents installation. Planned in new construction no problem, here in old buildings it can only be realized with very high effort. Incidentally, in the past a drain with a plug and corresponding slope was often installed in the middle of a bathroom.