tamte

POPs as a dedicated context were deprecated somewhere around H13
the new POP nodes are actually DOPs, so you should look up examples of DOP solvers and how to modify geometry data in DOPs

the docs mentions just gl_wireframe and gl_lit
would be great if docs reflect all the attributes, since it's very difficult to find out about them and there is more for sure, like gl_showallpoints

while there is no single answer to your question as it depends on various decisions
if you simply want to have a single Object which contains geometry of all the objects you've already created on object level, you can just use Combine tool from Modify Shelf

or just select your objects, hit TAB with your mouse over object level viewport, type Combine, enter