Can you attach external storage to a phone?

To answer this question yes, yes you can but it depends on the external storage. I am using a sandisk portable ssd that is NVME. Very low power and has the connectivity built into the ssd. If you were to use a flash drive you would need an adapter to make your mobile device recognize that a flash drive is attached to it.

But it is possible.

Say you take a ton of pictures and you back them up on google. You are paying a subscription fee for them to keep you alive on the internet. Depending on the storage you purchase it can get expensive.

Say you had 10 terabytes of pictures, google would charge you a thousand bucks a year to keep you alive online.

At the time of purchase I paid 129 for this sandisk drive. If I run out of space I buy another.