Installing a fiber glass shower in a roughed in 2 year old house (unfinished basemwnt)

The abs pipe is sticking up about 18 inches above the concret floor

Is it vented and does it have a p trap already installed its only 2 year old house

How do I cut the pipe at the right level and lift the shower over it so it fits right in the drain hole at the proper height

The water supply lines are easy it is pix pipe do I connect the lines before lifting the shower in place. Thanks.

Any other fixtures in the basement like a sink or toilet?

Is it vented and does it have a p trap already installed Souynds like you have a trap and if there's other fixturesyou are probably wet vented.

how do I cut the pipe at the right level and lift the shower over it so it fits right in the drain hole at the proper height
Your shower base should have detailed instructions on installation.
Good luck, Tom

If I cut the cap off the abs it will give us a sewer smell if it doesn't have a p trap under the cement floor, if no smell then there must be a trap

A better way to check for a trap is to remove the cap and pour a gallon of water down the pipe.
Now take a stick andm measure the water left. You should have at least a inch or so of standing water. I'd be willing to bet you're trapped.
As for the vent, please answer my question.
Any other fixtures toughed in the basement like a sink or toilet?
Regards, Tom

Yes there is toilet pipe sticking out also and a large abs pipe coming from up dtairs to use for a sink

a large abs pipe coming from up stairs to use for a sink That pipe would be the vent for the basement bathroom group. You should also have a pipe coming up through the floor to pick up the lavatory.
Sounds to me like you're trapped and vented after you finish off the group. Here's a typical bath group tough in, (see image). Good luck, Tom