ᐅ Shower Instead of Bathtub: What to Consider?

I’m planning to renovate this bathroom from the 1980s.
Mostly it’s tile over tile, and the toilet has an integrated cistern.
The bathtub will be replaced by a walk-in shower. For this, the faucet will also need to be installed higher. What is the item inside the red circle? It looks like a vent or an overflow. I’m not familiar with this system…

What was new for me and proved worthwhile in our case is a walk-in shower with a level floor. The advantage is that you can simply walk in without having to step over a threshold, and—less often, but no less useful—any water, whether from a burst pipe, a broken washing machine, cleaning, or other causes, will always drain here first.

The walk-in shower mainly depends on the existing drainage system. A drain that is positioned too high will prevent its installation.
In new constructions, this is usually not a problem, but in older buildings, it can only be achieved with considerable effort.

In addition, it was common in the past to install a drain with a plug and appropriate slope in the center of the bathroom floor.