Make a self propelled machine and a machine that cuts grass and smoosh them together.

We did two things first:

First we made a (simple) machine that cuts wheat stalks at ground level (scythe).

Second we made a (complex) machine that is self-propelling.

The first version is just a machine that mimics the human motion of doing that task. Its a mechanical reaper that is pulled by a horse:

https://okfronline.com/2024/02/inventions-of-agriculture-the-reaper/

Then someone figures out how to make a self contained engine and it becomes self propelled.

This is a process that occurred over centuries.

Well first there's the cutting technology. Scything is ancient. Industrial production of push mowers using cylinder blades were developed in the 1800s and the more modern gas powered rotary cutters came along in the 19th century with many other self-propelled and automated mechanisms. This started with coal and wood fired agricultural equipment used for processing, like threshing and winnowing. As internal combustion engines (gas powered) developed in the 20th century more equipment and processes could be incorporated on-site and in-field.

How does one even start to make a self-propelling machine that cuts wheat stalks at ground level?

You look at a horse-drawn mowing machine like this and start thinking about how you could do something similar without the horse.

Stationary processing of grain. Then stationary automated processing of grain. Then add a cutting machine (from harvesting hay) and self-propelling and you've got your combine harvester.

The loading thing? A extruder on a swivel, old tech.