So I was finally able to get my xpub key (tpub for testnet) with Bitcoin Core.
Note : it only works since v22 https://github.com/bitcoin/bitcoin/blob/master/doc/descriptors.md

Prune block storage to avoid downloading whole blockchain

Open Configuration File. Then add this to use testnet (testing blockchain).

File > Create Wallet (make sure Descriptor Wallet is checked)

Open the node window in Menu Bar : Window > Console

Then type listdescriptors (see first post's image)

You can read the unrolled version of this thread here: https://typefully.com/sbounmy/Dlpa6L3

Using Bitcoin Core to Obtain xpub Key (listdescriptors)