How exactly do all FPGAs work together on this board to carry out high speed calculations
The board simply provides an array of FPGAs and optional SDRAMs with "generic" connections among them as shown in the block diagram. As delivered, the board doesn't do anything at all (except consume power).

It is up to you, as the end user, to write HDL code for the FPGAs, either individually or collectively, that makes use of the on-chip and off-chip resources to solve whatever problem that you have.

Yes, you could make a very impressive graphics card out of this, but you would have to add a small amount of hardware (via the expansion connector at the top) to support the specific display interface you want to use.

To use this board, you would need to first get a (more) detailed block diagram of the resources on it, and then figure out how to "map" a hardware architecture for your problem solution onto it. You will need a copy of the Xilinx FPGA development toolchain; there's a free version, but for a project of this magnitude, you'll probably want to pay for the full version. You then write HDL code (e.g., Verilog or VHDL) to implement the logic you've mapped onto each FPGA. In some cases, you'll have identical (or nearly identical) code installed on all of the FPGAs, creating a mesh processor of sorts. In other cases, you'll create a "pipeline" of FPGAs, with each one working on a unique part of the problem.

You will also need to figure out how to get your data into and out of the board. You might do this by writing a driver for the host PC that does this via the PCIe interface, or you might interface the board to external hardware via its expansion connector.

Needless to say, any project involving this board is a major undertaking, but it's nice to know that it's there if you really need it. One application for this board is to test algorithms and their hardware implementations prior to designing a full-custom ASIC for mass production.