The dishwasher gap needs no special support. But it usually requires a brace to attach the dishwasher's tabs to, which is then covered by a trim.
- Install the lower cabinets, sink cabinet(s), the pantry cabinet and the fridge panel..etc. If you also install the upper cabinets, that's fine.
- Leave the gaps for the dishwasher and stove. Install a dishwasher brace.
- Buy the sink
- Call the installer to measure and determine what support is needed, if any.
- The installer will place the granite, glue/caulk it to the cabinets.
- The DIYer can install the under-mount or drop-in sink, silicone the counter edges, tile the backsplash etc..
A countertop that is 3/4in thick (2cm) may require underlayment, usually 1/2in or 3/4in plywood, to spread out the support around the cabinet sides it rests on, but it won't add much to the strength.
Another reason to add an underlayment is to visually build it up for a thicker look with nosing, and to avoid the extra weight.
A countertop that is 1.25in thick (3cm) usually does not require further support or build-up.
The price difference is often small enough that the thicker look is simply achieved by thicker granite rather than a lamination plus nosing.
The plywood has nothing to do with spanning the dishwasher gap: usually kitchen cabinets have no top, and the only support for the countertop is provided by the sides of the cabinets. Whether there is a dishwasher or a drawer underneath makes no difference.
Also, adding plywood on top of the cabinets is not going to close the gap between the bottom of the counter-top and the top of the dishwasher. To close that gap, you'd apply a trim, and treat it very well do withstand the steam from the dishwasher.
Different manufacturers have different requirements and practices, and whether plywood is added depends on the stability of the chosen granite and how it will rest on the cabinets. The origin of the stone and thus its strength matters, but I can't imaging that the country of installation would matter.
If the plywood is applied, nosing is required to hide the lamination edge of granite & plywood.
Your granite stone finisher will apply the plywood and nosing for you for a strong hold and finished look.
Bracing for the dishwasher space may be provided to ensure proper 24in sizing of the gap for it. The brace can be removed after installation of the granite. Or you can keep the brace in order to screw the tabs at
If plywood is required, you would generally not install the plywood for them, and instead leave the cabinets and dishwasher spot open on-top. Best to confirm with the installer.
If you are applying (granite) tiles for the counter top -which is a different matter-, then the plywood would be installed first, and then the tiles are laid on top.