How are custom metal objects fabricated or machined?
This process of fabrication requires a skilled machinist to interpret the drawing and produce the part. The pictured is a relatively simple one; a rectangular prism with a number of holes of a few types, but it is illustrative. One key thing to remember: any aspects of the part that are not clearly and specifically defined (with tolerance ranges) on the drawing or associated documentation may be left up to the discretion and assumptions of the machinist or the supplier he works for. Hopefully those assumptions align with your expectations for the part, but you will have no guarantee or recourse if they do not.
First of all, the block as a whole. The comment indicates that it is aluminum; this material requirement may be specified on some part of the drawing not shown. Even then, there are dozens of different specific alloys of the aluminum. If one is not clearly specified, the machinist may be able to freely select any of them, most likely the one that is cheapest or easiest to work with, which hopefully still meets the material requirements of the part.
There may be commonly-available stock matching the outside dimensions of the part, in which case it just needs to be cut to length. However, depending on the size tolerances and surface finish requirements, an oversized stock material might need to be machined down on all six sides to the final dimensions.
After the rectangular block is prepared, it will most likely be put in the vise of a milling machine, and the hole patterns drilled. The "on 25mm centers" description of the pattern will need to be translated into individual X and Y coordinates for each hole. If using a manual mill, some combination of incremental coordinates may be used, but for programming a CNC mill everything will likely be converted to one global coordinate system. The program (typically written in "G-code") gives the actuators a list of X and Y coordinates to go to, a Z coordinate for the depth of cut, and specify which tool to use.
For the M6 threaded holes, there will need to be at least two steps, a drilled pilot hole and a tap to form the threads. A pilot drill to more precisely start the hole and a countersink to smooth the edges of the holes may also be used. The plain holes will be drilled, and then a counterbore bit can be used to provide the indicated clearance for the screw heads. If the part designer chose wisely, the dimensions of these features will correspond with commonly-available industry-standard tools, unless there was a very good reason for them not to.
After machining is complete, the part should be de-burred (have sharp edges and burrs removed), and then it can go for surface treatment like anodization. If the finish process will add significant thickness (such as painting or powdercoating) that thickness should have been compensated for in all of the machined dimensions.
After all of the processes are complete, the finished part should be measured and checked against the specification drawings. The drawing shown here does not actually call out any tolerances on the dimensions. These may be applied drawing-wide in a title block or by some other convention, but if not it may fall under the "machinist's discretion" and any number of discrepancies would be allowed.
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