How to run `clang-apply-replacements` on change set generated by `clang-tidy --export-fixes`?

Command clang-tidy --export-fixes=fixes.yml <sources> generates a file fixes.yml with stances like

  - DiagnosticName:  modernize-loop-convert
      Message:         use range-based for loop instead
      FilePath:        'Base/Util/StringUtils.cpp'
      FileOffset:      1510
        - FilePath:        'Base/Util/StringUtils.cpp'
          Offset:          1514
          Length:          37
          ReplacementText: '(const auto & item : items)'
    Level:           Warning
    BuildDirectory:  'tidy/Base'

which looks entirely reasonable.

How to apply these changes to the sources? man clang-apply-replacements says

USAGE: clang-apply-replacements [options] <Search Root Directory>

None of the options are pertinent. Some web sources tell us we have just to put the change set into the root directory, which will be searched for any .yml file.

Doesn’t work for me: clang-apply-replacements . executes instantaneously, prints no message, and effectuates no changes.

clang-apply-replacements searches for .yaml files, but your clang-tidy command creates a .yml file.
Afaict, your commands should work as soon as you name your file fixes.yaml