How to fully decode a coinbase transaction?
coinbase, scriptPubKey, scriptSig - they are all scripts. You can read in wiki about how to decode script https://en.bitcoin.it/wiki/Script.

Coinbase can contain any data, even invalid script. But typically, it contains valid scripts. Examples of decoding:

coinbase: 038abd07062f503253482f048725ee5208083865a409000000092f7374726174756d2f
03 - length opcode
8abd07 - data with length 03
06 - length opcode
2f503253482f - data with length 06
04 - length opcode
8725ee52 - data with length 04
08 - length opcode
083865a409000000 - data with length 08
09 - length opcode
2f7374726174756d2f - data with length 09

scriptPubKey: 76a914975efcba1e058667594dc57146022ec46560a63c88ac
76 - OP_DUP opcode
a9 - HASH160 opcode
14 - length opcode
975efcba1e058667594dc57146022ec46560a63c - data with length 14 (20 in dec)
88 - OP_EQUALVERIFY opcode
ac - OP_CHECKSIG opcode