How do old computer system decode new unicode charcters?

Question

I have been reading about character encoding and the concept is really confusing. Here is my understanding and appreciate if any one can correct me if I'm wrong:
Unicode is a encoding standard which can represent characters beyond 8 bits. Unicode is yet reach its full size.i.e the last character thdat can be represented in Unicode.

Early computers were designed to understand ASCII but later unicode was integrated to new systems. So here is my doubt: if unicode set is continuing to have additional characters (emojis etc) how do old systems interpret them if they don't understand those characters?

Answer 1

You just see question marks or boxes instead.

𐘀𐘁𐘂𐘃

Chances are you don't see these characters above, it's from linear A, a writing system from over 3000 years ago we have not decoded yet.
Unicode defines it but I haven't seen any device that implements it yet.

However since you usually only need to update some fonts, even older systems should be relatively easy to update.