I noticed, that during service of my 2019 C300 a small lithium battery (CR2032) was replaced.
Does anybody know what this battery is for, where it is in the car and how to replace it?

Thank you much for any insight into this

That is for your key fob. What a generous dealer to replace that $1.99 item! YouTube videos cover the replacement.

Thanks, Smiles, that makes a lot of sense.
As far as "generous" is concerned, they charged $ 8 for it.

If you bring both keyfobs, they replace two.

Where I live its courtesy they replace these for free

this coin-cell service is included in annual A /B $800 oil change.

$8 charge is not out of line with troubleshooting+ repairs.