Use Kustomize replacements to propagate a field value from one resource to another

Steps

1. In kustomization.yaml, add a replacements: block (inline or via a path: to a separate replacement file); do not use the deprecated vars: field for new configs.
2. Under source:, use group/version/kind/name/namespace selectors (only as many as needed to match exactly one resource) plus fieldPath (dot-separated, defaults to metadata.name) pointing at the value to copy, e.g. fieldPath: data.API_URL.
3. Under targets:, list one or more {select, fieldPaths} entries — select uses the same GVKNN fields, and fieldPaths lists destination paths, using bracket/index syntax for list elements such as spec.template.spec.containers.[name=app].env.[name=API_URL].value.
4. Add options.create: true on a target if the destination field may not already exist, and options.delimiter/options.index for partial string substitution (e.g. splitting a value on '/' and replacing just one segment).
5. Run kustomize build . (or kubectl kustomize .) and inspect the output to confirm the value was substituted; select/reject in targets can match either the resource's original name or its post-transform name (after namePrefix/nameSuffix).