[GRASS-user] Simple example about vector database layers in GRASS

[roy roy]

I'm reading this wiki page:

https://grasswiki.osgeo.org/wiki/Vector_Database_Management#The_concept_of_layers

about the GRASS vector model ad the sketch seems to suggest that I can 
setup and display
several layer given an available or digitized vetor geometry

In the example I have a vector object (centroid)

id = 16 ( a centroid )
(this id is automatically assigned an internal feature id ... can the id 
be diplayed?
how do I know a geometry id?)

joined with two distinct attribute tables via category values, i.e.

16 -> 31  (wheat)
16 -> 26  (a name)

then there is a boundary with id 1:

1 ->  5 (connected to category 5)

what it seems not clear is how you set up such connections:

How do i know what id is assigned to a geometry?

How do i connect "category" to "id" so that I can assign an owner to 
each area ?

what does the following phrase from the page means?
"code" value 34 within table "plots" has no corresponding category in 
layer 2;
is the term "category" used instead of "id" (quite confusing ...)

is there a list of modules to perform this operation that could help to 
better understand?

(eg. use v.category, then use ...)

thanks for any help