How to use Snap to assemble objects together Firstly I am using v2.92 and have viewed the suggestions listed after I entered the title in which I could not find an answer.
I have now, at last, created all the parts to my VERY FIRST 3D object (floating dock) together with UV mapping of my preferred texture and am now ready to 'assemble' all parts into a single object. I did receive information before about 'snapping' and was advised to use Snap &gt; Increment  with Absolute Grid Snap 'ticked' which I can't find anywhere to even attempt to try.
Refer to the separate objects in my image below. All objects have a scale of 1 for each axis.
A is the dock created with an array of 2 so that I could offset textures to simulate 2 separate planks along the side.
B is an array of narrow planks which is for the walkway on top of the dock
C is an array of supports which will go on top of the walkway to support a timber rail. Before I position these I will add another array on the X axis so that there are arrays on both sides of the dock rather than duplicate and have to position 2 separate arrays.
D is the timber rail - I will also add an array on the X axis for reasons explained above. I left those array out of the image for simplicity for the image.
I seem to have successfully moved B on top of A with a bit of difficulty but want to ensure there is no 'gap' between each.
I cannot get C positioned to be on top of B - it just disappears when positioned over the dock
Is there an easy method to achieve what I am trying to do and/or suggestions as how to best do what I am trying to do.
This is no solution how this all will completely come together, just some hints how things work to maybe achieve it.
First of all, Absolute Grid Snap, you can find it in the Snapping dropdown menu.

To use snapping, you have to enable Snap (the magnet to the left of the Snapping options. This way snapping is used whenever you move the object (or rotate or scale if you checked that, too). If you don't want permanent snapping, disable the magnet and hold Ctrl when moving objects for temporary snapping.
Now, in Object Mode the origin point is snapping to the grid (if Absolute Grid Snapping is enabled), for example X=1, X=2, X=3 etc. or to to full increments of Blender units relative to the starting point of the movement (e.g. move 1m, 2m, 3m).
Now to position your elements where you want them with Grid Snapping you have to make sure that their origin points are positioned so that this might work and that the objects' dimensions are maybe full increments of the grid as well.
Another way would be setting the Snapping to vertices, edges or faces and try if this works better for you. With vertices enabled, one corner vertex of a cube snaps to another corner vertex of a cube, no matter where their origins are or how they are positioned on the grid.
The only problem with this is that (at least I) sometimes don't know how to tell Blender which vertex I like to snap to which other. By the way, if you limit movement say to X direction, you can make vertices snap to line up with another on X, but not fall together in the exact same position.