Not an aeronautical engineer, but understand that the cylindrical shape used in today's aircraft design is very good at containing the stresses of a pressurised cabin.

However, the design does mean significant pressure stress on the front and rear of the cylinder. This must be managed with extensive (heavy) structures.

A doughnut design, in theory at least, solves for this by removing "the ends" of the cylinder.

Initially I thought the "hole" was meant to be enclosed within the airframe, but it is there. See Fig 2 - Fig 8 in the patent application. They have even diagrammed a method for boarding passengers using retractable escalators from parallel hatches located on the forward and aft planes of the inner-torus, as well as using an elevator to board passengers from within the hole! Fig 9 - Fig 16 diagram the invention applied to a more conventional airframe (without the hole).

The patent title is "aircraft including a passenger cabin extending around a space defined outside the cabin and inside the aircraft." My understanding is that the patent covers the torus-shaped passenger cabin within any larger aerodynamic envelope as a method of reducing constraints related to cabin pressurization (the "hole" is incidental). Very interesting stuff. I'm not an aeronautical engineer or patent attorney.

Paragraph 0030 says that figure 2 is without the aerodynamic envelope. The aerodynamic envelope is labeled 22, and visible in figure 7. Other parts of the description mention how the hole can be filled with cargo or other items, and that a non-pressure skin can be used as a cover.

I think the donut hole would be used as unpressurised cargo space for luggage, moving the floor of the cabin closer to the bottom of the pressurised envelope, and reducing the strength and fastening requirements of having a pressure bearing floor separating the pressurised cabin space in the top half of a cylindrical aircraft fuselage from an unpressurised cargo space below.

I don't think any aircraft are built that way though, so I'm a little confused.

Layman here. Looks to me like the outside of the aircraft would remain solid, while these 'doughnut walls' would be strictly internal to help with airframe strength.

Another layman here. In claim 1 it refers to "a space outside the structure" which to me implies this is literally a hold right through the aircraft. EDIT: I think I'm wrong here, the structure refers just to the cabin.

Figure 2 shows it without the aerodynamic envelope. Figure 8 shows the cross section. There are hatches on the bottom which open up for loading and unloading. These can be used for stiffening, but don't need to withstand pressure so can be weaker.

In paragraph 0085, this interior unpressurized space can be used for luggage or freight. Or (paragraph 0094) to store the spiral staircase.