outdoor wooden garden bridge

What's the china fir? is it fir wood?

no, it is not a kind of fir wood though it called china fir wood, its genus name is Cunninghamia honours, for its details, pls see the blow description in Wiki.

/wiki/Cunninghamia

What's the water-based paint

it is a kind of environment paint which used to stain color, we do it by dig

How long it can keep well in outside?

at least 5 years without any protect, china fir wood has great ability in ourdoor, pls don't worry