All major operating systems have their own file-sharing protocols (AirDrop, Nearby Share, etc.) which are incompatible with each other. There do exist apps for "cross-platform file sharing", however, they require installation on both the sender and the receiver devices, which makes it a bad solution for quickly sending files to other people.

I was thinking about making a file sharing solution that acts as a bridge between those different protocols and only requires action from one of the parties. However, there's a huge issue with AirDrop - it uses a custom networking protocol called AWDL. There are open implementations like OWL, but they still require low-level wifi access, so it can't be run on Android without kernel modifications.

This means that the only way this can be implemented is by using a physical device, eg. a Raspberry Pi Zero W or a similar microcomputer that runs a custom firmware. An obvious problem is that it requires a lot of effort - you need to buy a Raspberry, flash it with the firmware and carry it around with you, just to be able to send or receive files to other devices without using third-party websites.

So I'm personally not sure if that's worth making. But maybe you'll still be interested? Is this a big enough problem for you to want to use a hardware solution?