Some Fossies usage hints in advance:

1. To see the Doxygen generated documentation please click on one of the items in the steelblue colored "quick index" bar above or use the side panel at the left which displays a hierarchical tree-like index structure and is adjustable in width.
2. If you want to search for something by keyword rather than browse for it you can use the client side search facility (using Javascript and DHTML) that provides live searching, i.e. the search results are presented and adapted as you type in the Search input field at the top right.
3. Doxygen doesn't incorporate all member files but just a definable subset (basically the main project source code files that are written in a supported language). So to search and browse all member files you may visit the Fossies civicrm-core-5.59.3.tar.gz contents page and use the Fossies standard member browsing features (also with source code highlighting and additionally with optional code folding).