Apple - Global convert to uppercase and lowercase in macOS?

You can do it in Safari, but Chrome ignores the standard Mac Toolbox, unfortunately.

System Prefs > Keyboard > Shortcuts > App Shortcuts

You can set key commands for All Applications, which will then work for all that have the requisite menu option.

This is my own setup, which shows keys already set for Capitalise & Lower Case
I use it a lot for such as Stack Exchange.

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This free app has worked on all Mac versions for over a decade and is still current for Catalina 10.15:

WordService

WordService provides a large number of commands for working with selected text. Extend your favorite word processor, email app, or web browser e.g. with functions for reformatting text paragraphs, cleaning up tabs, quotes, or line endings. Remove unwanted text parts, sort lines or paragraphs, change case, and do much, much more with text.

  • https://apps.apple.com/us/app/wordservice/id899972312?mt=12
  • https://www.devontechnologies.com/apps/freeware