What is the Witness and what data does it contain? I've learned about the Segregated Witness but couldn't find info on breaking it down. 

I know usually when legacy addresses are used the public keys for example will be under Scriptig.  With P2SH, I can see that the ScriptSig contains the hash of the redeemScript which must match the hash of the address in the Output ( I think). But I am not sure about the witness data.

Let's take this TX:
80975cddebaa93aa21a6477c0d050685d6820fa1068a2731db0f39b535cbd369

What info is in the witness, can someone break it down please?
This structure contains data required to check transaction validity
but not required to determine transaction effects. In particular,
scripts and signatures are moved into this new structure.
The witness is a serialization of all witness data of the transaction.
Each txin is associated with a witness field. A witness field starts
with a var_int to indicate the number of stack items for the txin. It
is followed by stack items, with each item starts with a var_int to
indicate the length. Witness data is NOT script. See BIP 141.

Witness data depends on the type of transaction.
Input 3
Looking at transaction input #3 in the raw json:
script = a914771962306e72e479245d48e879dd2a1862225b4c87
This has the structure of a Pay to Script Hash (P2SH) transaction. Because of the witness data, this is probably a multisig P2WSH nested in BIP16 P2SH:
witness:      0 &lt;signature1&gt; &lt;1 &lt;pubkey1&gt; &lt;pubkey2&gt; 2 CHECKMULTISIG&gt;
scriptSig:    &lt;0 &lt;32-byte-hash&gt;&gt;
              (0x220020{32-byte-hash})
scriptPubKey: HASH160 &lt;20-byte-hash&gt; EQUAL
              (0xA914{20-byte-hash}87)

witness:
04 (4 stack elements)
00 (1st stack element, 0 Bytes)
47 (2nd stack element, first signature, hex: 0x47 decimal: 71 Bytes)
304402202c3f94e5daf4057377d9f16d45b57e962de42fb42cb7e95a0382b7c66624980a02204098f6acd43b0391ea1b4a8102797e78895848fb7e883f98d207d14d45945a6901
47 (3rd stack element, second signature, hex: 0x47 decimal: 71 Bytes)
30440220448460edd5291a548c571ccf3a72caf47b02364035dc84f420d311e3a0c5494802205bb1cc89f20dc1e2c1f6eadb74898f8eecc46fbf488b676636b45fafaeb96e0f01
69 (4th stack element, 2-of-3 multisig script, hex: 0x69 decimal: 105 Bytes)
&lt;52 OP_2&gt;
21  (33 bytes)
&lt;021e6617e06bb90f621c3800e8c37ab081a445ae5527f6c5f68a022e7133f9b5fe pubkey1&gt;
21  (33 bytes)
&lt;03bea1a8ce6369435bb74ff1584a136a7efeebfe4bc320b4d59113c92acd869f38 pubkey2&gt;
21 (33 bytes)
&lt;0280631b27700baf7d472483fadfe1c4a7340a458f28bf6bae5d3234312d684c65 pubkey3&gt;
&lt;53 OP_3&gt;&lt;ae OP_CHECKMULTISIG&gt;
Note that the OP_2 and OP_3 indicate this is a 2-of-3 multisig transaction script.  See Pay-to-Script-Hash | Transaction