The tray (lounger) can be used for a long time. We had the baby in the shell (lounger) for so long that it didn't want to lie down anymore.
I think up to 6/7 months, size 74/80. Every baby grows differently. But no child wants to be in the shell (cot) for that long.

There is no newborn insert for the car seat. It has a car seat up to 9kg. It has to be in the seat.
We have used the baby car seat from Newborn 48cm. up to 9kg. Size 80 used. After that, the head peeks out the top, which is not ideal.