If you turn on multiple locations in SHOPIFY it is not supported by the retail express at all. Or our connector without configuration. We only see the stock according to the source group.

Multiple locations should not be turned on in SHOPIFY as this can’t be supported by the integration and the other locations will always show zero.
Every Product in REX has a Sales Channel and a Source Group attached. This source group is what the order then uses for fulfilment.

Please refer to Retail Express article Source Groups Overview