This is what post boxes locally look like - bright yellow, and typically adorned with all kinds of stickers accumulated over time. Two slits to insert your snail mail into: same city, right slot, all other places, left slot.
The specimen in the close-up has had specific instructions added to it: "love letters only", the rightmost slot is right for that 💛🧡. We all seem to need a reminder of that kind these days! I smile when I walk past this one.
Other one, if you look closely, a custom sticker plays a (mildly interesting) pun that does not lean itself easily to an explanation. Berlin is somewhat poor (and cheap) in comparison, and the area where I live has received over the last fifteen or twenty years or so immigration from the affluent southwestern parts of Germany: left slot, relabeled to indicate mail for "away places" in the old southwestern home of the newly arrived.