There is not enough information in the drawing to give a definitive answer
The question effectively is, which of the three highlighted surfaces should serve as the "start point" for the "76 long" dimension, given that we can't see the back side of the part.
In my opinion, it should be the very front face of the part.
My reasoning for ...
I'm modelling this in SolidWorks, because it's what I have installed on this PC. Everything should be transferrable to Inventor, but ask if you have any questions.
Step 1 - create the central core. N.B. the '8mm' here is a driven dimension. The drawing shows 8, not 8.0, thus I'm inclined to thing the marked 8mm is a rounding error. Placing the hole ...
I'm answering your direct questions in this post, and will follow up with a how-to-model as a separate answer.
How do I position this main plate in terms of depth from the
reference plane to the near face of the plate? What looks like a "51" is actually a "5" and a line, dimensioning the depth of the hidden detail cutout (a mirror of ...
By default, the AutoCAD shall start at the origin point (x = 0, y = 0, z = 0), unless you are using "user coordinate system (UCS)" defined by the previous user. You can delete or modify UCS by following the steps indicated in this article.
Or as others have indicated, you might need to redefine the "Snap" command features.