yq/pkg/yqlib/doc/headers/File Operators.md

File operators are most often used with merge when needing to merge specific files together. Note that when doing this, you will need to use eval-all to ensure all yaml documents are loaded into memory before performing the merge (as opposed to eval which runs the expression once per document).

yq eval-all 'select(fileIndex == 0) * select(filename == "file2.yaml")' file1.yaml file2.yaml