How to transfer SSH keys from a client machine correctly?

Question

linux

Set up access via keys to a remote server (Ubuntu 18.04). The client machine at the time of setup is Linux (Debian 10). At the same time, I will constantly rush between different computers - Linux, Windows. And still a lot of time only with Android.

How to import the keys that are currently on the client computer to these devices? Or a banal transfer?

Answer 1

Never do that.
The private key must not leave the machine on which it was generated.
Generate a key pair on another device and add the public key to the server, each device must connect with its own keys.

Answer 2

Banal transfer and conversion of a private key into a format understandable to the application.

Answer 3

As a fallback: copy from the server on first connection and check the hash.