How do internet wallets connected to full blockchain nodes How do internet wallets such as Coinbase connect to the a blockchain node? Doesn’t the node itself create a default wallet object on startup? How is this replaced by internet wallets which hold multiple address until account credentials.
How do internet wallets such as Coinbase connect to the a blockchain node?

Using an API provided for that purpose. For example Bitcoin core's API

Doesn’t the node itself create a default wallet object on startup?

Not all nodes are based on Bitcoin core. Apart from the development costs there is nothing that prevents exchanges writing their own software to act as a Bitcoin node.
There are ways of running a node that don't create an unwanted wallet. (e.g. set disablewallet=1 in the bitcoin.conf file.)

How is this replaced by internet wallets which hold multiple address until account credentials.

In proprietary ways that are likely different from exchange to exchange and which are likely company confidential.