.publicfile. You can safely share your public key via an insecure channel (e.g. via an unencrypted messaging app, like Discord).
.privatekey files to external storage (e.g. memory sticks). The default key folder depends on your operating system:
.zip, etc) that is at least 64 bytes long can be used as a keyfile but using an ordinary file type is riskier since it is more likely to be accidentally modified. Files high in entropy, like those listed above, are strongly recommended if you do not want to use a randomly generated keyfile. Text files should generally be avoided.