How can I host a local WiFi network which will show up in an Android's network manager?

I am trying to connect an NVIDIA Jetson Nano with an Intel Dual-Band Wireless-AC 8265 over WiFi directly to an Android smartphone. I can make the devices connect through an intermediary wireless router, but this system needs to be able to work independently out in the field. I'm not looking for an access point or internet forwarding, I just want to be able to connect up the sockets on a pair of apps, one on the Jetson and one on the Android, so that I can send UDP packets.

I have used several devices which have this functionality, where as soon as the device is turned on it begins broadcasting availability of its own local WiFi network. A nearby device can see this network (it shows up in the network manager), authenticate, connect, and begin sending and receiving data all wirelessly with no intermediary. This is what I'm looking to accomplish here.

The research that I've done indicates that Android devices cannot connect to ad-hoc networks, which would explain why I never saw the Jetson's ad-hoc network in the Android's network manager. I don't have any option to create a local network in "infrastructure" mode, as many people seem to think is necessary. If this is the case, a lead on a WiFi chip which supports this would be useful as I've had no luck in finding one that explicitly mentions the functionality myself.

So, to reiterate, how can I achieve this direct connection between a network hosted on an Ubuntu machine and an Android smartphone?

It's not exactly what you're asking, but it might get you the same result (which I presume is to let the Android device access files on the Jetson, or vice versa).

This article gives information on how to set up peer-to-peer networking via a device hotspot -- which should, with the right installed software and settings (SAMBA on your Jetson, something equivalent for Android) allow you to share filesystems from one to the other.

Short version: you set up a hotspot on the Android device (with previously installed software that doesn't just share the Android's data connection), and use previously installed software on both the computer and the mobile device to share files from one to the other. The linked article is probably aimed at Windows for the non-mobile end, but it's highly likely there is Linux software that has the same function.

This is similar to setting up a routerless peer-to-peer network with a crossover Ethernet cable, in terms of functionality -- and most importantly, from your question, it is independent of whether there's cell signal at your remote location.

When creating the network on the Ubuntu machine, set it up as a HotSpot network, rather than ad-hoc.

This might be unintuitive if you specifically do NOT want access point / HotSpot / internet forwarding behavior, but it seems to work. An android (or any other) device near the host will see the network in the standard network manager utility and can connect with an optional security key, although the Android may warn you that this connection doesn't actually provide you with any internet. You can also choose to set a static IP address for the host machine on the network, meaning the client socket programs can be certain where to send their connection requests to.