How segwit blocks are seen by segwit and non-segwit nodes?
There aren't two part to a block like you describe. The word segregated in segwit means that the witness data isn't included in the TXID computation similar to how it always wasn't included in the signature hash computation. It's still included in transactions and blocks. 

With segwit, the signatures are in each transaction in a witness field that is included before the nlocktime field.

When a node relays a segwit using block to a peer that doesn't understand segwit, it strips the witness fields out of all the transactions that have them.  Since they don't have any rules pertaining to segwit data they're perfectly fine with this.

If you were to hand a stripped block to a non-outdated node, it would just drop it and ban you for giving it invalid data-- it knows about the segwit rules and attempts to validate the additional data.