MacBook Pro 2015 Storage Count

I am going to be replacing my MacBook Pro Early 2015 with a MacBook Air M3 in the next few weeks and was checking the storage on my previous laptop so that I will have enough on my new one.

When I looked at my storage it said that it was using 500GB flash storage as opposed to the 128GB, 256GB and 512GB options I thought were available for the MacBooks of that time. I am not the first user of the laptop. Does this mean that the SSD was changed?

I am not too good with understanding of computer specs/parts but I thought Apple made it quite hard to change parts especially storage because they charge a lot for it.

So basically I want to know:

• What does the 500GB Flash Storage mean?
• Would it be possible for me to change the storage of my M3 MacBook Air after purchase for cheaper than the upgrade?