I start by testing calling a REST API from Postman : this works.
I then set up a Salesforce "External Service" and call the same service from a Flow : this works.
Then I create a LWC component which does the same thing with a fetch callout : this does not work, complaining about a CORS problem.
How does Salesforce External Services manage CORS ?
Why does calling the same REST API from Salesforce External Service and Salesforce LWC fetch not react identically regarding CORS ?