How do you use Greek letters in pages, 5.5.2?

Open the Character Viewer and drag the letters from there in to your document.

If you have to use it more frequently you can set it up bit automated, in the syst pref- Keyboard- Text. Then assign a Greek letter to some word like when you type Beta it would be replaced with Greek β.

In System Preferences > Keyboard > Input Sources, add one of the Greek Input Sources.

You can then select that input source from the menulet that appears in the right hand end of the menu bar.

You'll have to work out which letters on your keyboard add which Greek characters, but you can use the Keyboard Viewer (a menu item in the menulet) to choose and type the chars.

https://support.apple.com/en-gb/guide/mac-help/mchlp2214/mac