But now A, the unknown, black-box, non-Unix/Linux device has to relay between C and B. Why would that functionality be available on A?

If A is doing (or can do) ad-hoc networking, then the only problem is can B share internet over an ad-hoc network.

Why can A not be on the Network that C uses? Abd I would like to congratulate the OP for the proper use of a Picture.

@A.B The simple answer here leaves us with two options: 1). A is configurable and can be put on the same network as C, or 2). C must be bridged to B, so that both A and C can Share B. Option 1 Removes B from the picture all together. Option 2 makes B the gateway Device. The OS is irrelevant in both options as this is a networking question only unless option 2 is used.

@eyoung100 A is not configurable, it is a purchased device and I cannot modify anything. A emits a Wi-Fi signal and to receive data it is necessary to connect an Android device (C) to A's Wi-Fi network in order to receive data from its application. If A could connect to a hotspot issued by C there would be no problem since A and C would be connected and C would maintain an internet connection. However, A's app does not support that functionality and the only way to receive the data captured by A is to connect C to the Wi-Fi created by A.

@eyoung I would like to know if I can add another device (B), with an OS that does support simultaneous connection of network interfaces (for example Ubuntu) and share the internet through the Wi-Fi created by A.

@WorkPhoneID002 Can B see A's Network?

@eyoung100 Yes, B has an internet connection (via a 4G dongle) and is connected to A's Wi-Fi.

Can C connect to B independent of A?

@eyoung100 Connect to B how? Creating a hotspot in B?

Any Android Phone has the ability to connect to Wireless. If you disconnect C from A, can you reconnect it to B?

@eyoung100 Sure, if I create a hotspot on B I can connect C to B, but the goal is to receive data from A...

If A can talk to B, and C can Talk to B, then A can talk to C through B. The goal here is to get all the devices talking in the same subnet/class, i.e., all the devices have an IP Address of 192.168.1.0/24. If you create a hotspot on B, does it disconnect A?

Are you creating the Hotspot on your Phone or From the Pi?

@eyoung100 I am creating the hotspot on the raspberry (B), when I turn it on a sign appears indicating the Wi-Fi connection to A will be disconnected while the hotspot is on.

Thats a limitation of the Pi. These comments are getting a bit long. Try this: Connect C to A. Create a Hotspot on C and let me know if it Disconnects A. If the Phone hotspot and the Phone wireless can't be on at the same time either you have no way to get these to talk. Marcus is correct. If we can't configure A after trying a Hotspot from C, you might be out of luck. It might help us all if we knew what the device was. Being that it's on your Private network, no one can get to it from here.

@eyoung100 If A can talk to B, and C can Talk to B, then A can talk to C through B. Theoretically, yes. Practically, there's a lot of potential for problems with broadcast domains, and other infrastructure-type communications. But if the app on C refuses to talk to B, then that theory won't take you anywhere.

@MarcusMüller I was going to tackle that one after we got everything connected to B, which failed anyhow.

@eyoung100 I don't think there's anything to tackle, unless you can start to modify that app. A device configuration app refusing to talk to anything than your smart lightbulb's / your doorbell's / your PV inverter's/ … own wifi sadly is pretty common.

@MarcusMüller Sad, but a cheap way to implement security. Locking down the network prevents what your answer discusses "firmware quality... botnets..."

@eyoung100 Device C does have the ability to connect to A and also create a hotspot. Also, if B cannot create a wifi point while connecting to A due to hardware, I can add a second wifi adapter.

OK, I won't write an answer for this, but if the Pi and the IoT device can connect through C, then the Pi can get to the internet using your phone's data plan. adding a second WiFi adapter will work, as long as you create the Pi hotspot first then enable the 2nd adapter. After you have the 2nd adapter reconnect everything to B, as we discussed previously. The reason I'm not writing an answer though is because I quite agree with Marcus, i.e., if the IoT device is a camera or lightbulb hackers can now see you or turn on or off lightbulbs

@eyoung100 In reality the device is neither a camera nor a light bulb. In the graph I have added a camera to try to make the example clearer since it is a very common case. Device A is a fishing echo sounder, specifically a Deeper Chirp model.

See my answer below and adjust accordingly. Since we got all this worked out for you, I hope you can catch plenty of fish

@eyoung100 "The OS is irrelevant in both options as this is a networking question" it is relevant: see unix.stackexchange.com/help/on-topic . While "The underlying *nix OS on an embedded system" is on topic. If it's not about such underlying OS it can get off topic. Even Android can be off-topic if it's not about using it like an *nix system (else it has its own stack: android SE).