Answer 1

MB and mb are two diff things
MB =mega bytes , mb = mega bits
1 GB = 1024 MB
1gb = 1000mb

Usual hard drive contains 512 GB which is like 512*1024 MB which is huge for normal people but not for people like prof. programmers

Answer 2

Nathan Cole somehow Sololearn includes the comma in your link, so it messes up the hyperlink address. Here it is without the extraneous comma:
https://www.orangehardwares.com/blogs/guides/mb-vs-gb-difference-between-megabyte-and-gigabyte