You will need a film changing bag or tent to properly change film in the field. For example, https://bhpho.to/3gHObwO

There is no film holder needed for this camera, as you infer. The holders listed under accessories, are listed in error.

No film back needed , you mount the 4X5 film in the box

The closest paper you can get the Ilford Multigrade IV RC Portfolio Paper (Glossy, 4 x 6", 25 Sheets), BH # ILP1K4625S. https://bhpho.to/3m70iDf

You can choose to expose either film or paper, or use the paper for enlarging/printing/contact sheets. It's totally up to you! With the Ilford Obscura Pinhole camera, you can use the provided 20 sheets 4x5" Ilford Multigrade IV RC Paper to make paper negatives. The pros of shooting with paper is that you can load it in a darkroom with a safelight (easier loading), but the cons are pretty significant, such as lower ASA, limited tonal reproduction, and a more difficult reprinting workflow.

If you want to develop it yourself, search online for a PDF from Ilford called Processing Your First Black & White Film . You'll need a darkroom. Also developer, stop bath and fixer; all three chemicals can be purchased from B&H. A developing tank is handy, but not required. You can use glass baking trays to hold the chemicals and develop by hand (wear rubber gloves). But the room must be completely dark; the slightest light, even the dial on a wristwatch, will fog or ruin film. I started developing in a bathroom with thick black plastic covering the door. It worked well.If you want to print your negative, you can use the same stop bath and fixer, but buy paper developer. You can also use that to develop Harmon Direct Positive Paper, DPP, that should come with the Obscura kit. You can use a darkroom safe light, or red light, with paper. B&H also sells those. With a 4 x5 negative, you don't have to have an enlarger, just place the negative flat on the paper and expose it to light. This produces a contact print. Drop that in the developer and watch the magic.