From a detailed Neuralink reality check
"Before taking a look at the individual components that Neuralink announced recently, let's start with an overview of what the company hopes to accomplish technology-wise. The plan is to access the brain via a hole less than eight millimeters across. This small hole would allow Neuralink to implant an even smaller (4mm x 4mm) chip and its associated wiring into the brain. The chip will get power from, and communicate with, some wireless hardware located behind the ear, much like current cochlear implants.Elon Musk’s Neuralink: Both an evolution and a plan for radical change | Ars Technica
Inside the brain, the chip will be connected to a series of small threads that carry electrodes to the relevant area, where they can listen in on the electrical activity of neurons. These threads will be put in place using a surgical robot, which allows the surgeon to insert them in a manner that avoids damaging blood vessels.
The chip will take the raw readings of neural activity and process them to a very compact form that preserves key information, which will be easier for their wireless hardware to transmit back across the skull. Electrical impulses can also be sent to the neurons via the same electrodes, stimulating brain activity. Musk thinks that it would be safe to insert as many as 10 of these chips into a single brain, though Neuralink will obviously start testing with far fewer."