How convenient is it to use ssh on linux with key authorization with many servers and various clients?

Question

SSH

There are several servers on linux. Recommend how it is most convenient and safe to implement key authorization if you have to access them from various ips, from various operating systems (win, lin, android), from various devices (PC, smartphones) and sometimes completely random, from various clients ( putty, openssh, all sorts of anlroids)
Keep the private key on the flash drive? What about smartphones? Mail or Cloud?

Answer 1

Make yourself a separate virtual Linux and go to it.
And from it already set up all the keys and access to other servers.

Answer 2

Use port-knocking Then you only need a
ssh client