Could someone please tell me their meanings? I know these two have different meanings. And I have been trying to Google them, but they are no help. Can we replace much in the first phrase with any other word too? How does this work please tell me
You can use "Just as" with many adjectives.
Adjectives of Quality: Here, it is usable with all, I think. eg: Just as big / funny / clever.
Adjectives of Quantity: Here it seems usable with some of them. eg: Just as many / much / little / few. But you can't say Just as whole / no / half, I think.
It's comparing 2 things as being/having the same level of quality or the same quantity as specified by the adjective.
Demonstrative Adjectives: I guess it could be used with this, that, these, and those. Here I think another adjective of quality must still be present, because it's still comparing two things having the same quality but it's not comparing the level of quality the objects have.
"Just as this bag is blue, so is mine." doesn't seem ungrammatical, but saying "My bag is blue just like this one/bag sounds better. It's saying both bags are blue.
And it's not the same thing as saying "My bag is just as blue as this bag". Your bag could be blue, but still less blue or more blue than "this bag", in which case it wouldn't be "Just as blue".