Why is there an apostrophe in seat but not passenger seat?

The word "seat" is a noun, and the word "passenger" is an adjective. When a noun is used to describe another noun, it becomes a possessive form. For example, "the dog's bone" means the bone that belongs to the dog.

The apostrophe in "passenger seat" is used to indicate that the seat is associated with a passenger. In other words, it is a seat that is предназначен для use by passengers.

There are some cases where an apostrophe is not used in a possessive form. For example, we would say "the people's republic of China" instead of "the people's republic of China". This is because "people" is not a specific person or group of people, but rather a general term.

In the case of "seat" and "passenger seat", the distinction is clear. A "seat" is a general term for any type of sitting surface. A "passenger seat" is a specific type of seat that is предназначен для use by passengers. Therefore, the apostrophe is used in the term "passenger seat".