I'am learning about Bitcoin and I didn't understand something. Maybe someone could clarify. I [think] I understood what Segwit does (increases block size, solves malleability and works better with Lightning Network).

What I don't understand is how an increased block (4MB) is considered valid by old clients (without segwit)? Don't they verify the whole block size including here everything also signature (witness) that is segregated?

On blockchain explorer I see 3992.604 kWU and 800 transactions for block #512446. What is 3992.604?? Bytes? I understand the acronym kilo weights unit but what are those weights?

Thanks you