It is to drain residual liquid left in piping.
if your waste pipe is PVC and your glue is for PVC . yes! but make sure to use a primer(cleaner) on the pipe first.
There is no special PVC pipe. CPVC can be used for hot water applications or in my opinion just use PEX pipe.
I could hear the girl weep and sob.
Not sure what you're asking. Do you need to know size of piping or what kind of pipe to use?
The answer is yes, if the pressure is below 150-160 psi.
You can use pipe thread sealant aka "Pipe Dope" you can get this at most hardware stores.
On one job I had to run 64 lines up from the basement area
Secure the pipes. Use spray foam on the pipes, drain lines so no holes or leaks .
You can use epoxy resin to seal large holes or cracks in plumbing pipes because it is impervious to water and oils.
No, refrigeration pipes are normally quarter of an inch, this pipe size is far too small, use the industry standard, half inch pipe size.
Not directly without some type of sleeving or protection between them. They are dissimilar metals and it will create electrolysis which will severely impair the copper pipe and create holes in the copper in the future.
Galvanized pipe will be silver/gray color. Copper will be copper. Use a magnet, it'll stick to galvanized pipe but not to copper. Drinking water lines should not be black steel pipe.