reindexAll() :

This method is used to reindexed all indexer at once. It's ensure all data is up-to-date across the entire store. It typically use when making any bulk changes to the catalog ( Adding New Product, updating prices or changing attributes value). It can be resource-intensive process ( especially for large catalogs ), as it process all indexers regardless of whether individual need to updating or not.

reindexEverything():

This method was more aggresive approaches to reindexing in magento ( In 1.x ). It was designed to reindex all data, similar to reindexAll(), but it could also include additional processes/ checks that might not be present in reindexAll(). It was using to perform more extensive operations, depending on it's implementation. It was often used in scenarios where a complete refresh af all index data was necessary, including any custom or additonal indexing logic.

Note:

reindexEverything() was using in magento 1.9. But in Magento 2, the indexing system was significantly restructured and improved compared to Magento 1.x. As a result, the method reindexEverything() was not carried over into Magento 2.