The "best" way to add storage depends on your needs.
You can replace the internal storage with a larger capacity drive or other device. Some Macbooks can have an additional internal device added.
External storage will be a large variety - USB device. Ethernet connection. Thunderbolt devices.
Some examples:
Do you need that external storage to be portable, to easily take with you as you go?
Or, do you need a large amount of storage, with multiple drives, which could be in multiple enclosures, or a single enclosure where all of your external storage lives? That may or may not be easily portable.
Or, there's net storage, including having access to your stored data on the cloud. There's lots of options with that.
Some of the choices may not be practical for you - again, it depends on your needs.