First, you need to know about making a false door/false region (Find it on other topic, i have no idea how to explain this. Or you can look in the open source down there.)
Second, you need to know about moving door (or elevator) system, this is a basic knowledge of map making.
Okay lets start.
Spoiler: Show More
At the end it will end up like this (ignore every stuff i didn't mention on the list) : https://prnt.sc/hrx6d8
If you still doesn't understand (of what i'm talking about), try this map : aldien-re (Instant demo link : http://plazmaburst2.com/?s=2&map=aldien-re)
Map link (Clearly, Open Source.) : http://plazmaburst2.com/?s=9&a=&m=aldien-re&id=826584
I'm sorry if you still doesn't understand, you can PM me.