TYPO3 v8 has reached its end-of-life March 31st, 2020 and is not maintained by the community anymore. Looking for a stable version? Use the version switch on the top left.
You can order Extended Long Term Support (ELTS) here: TYPO3 ELTS.
Extbase / Fluid¶
For learning extension development with Extbase and Fluid you can use one of these resources:
online resource: Developing TYPO3 Extensions with Extbase and Fluid (8.7)
book: Michael Schams, Patrick Lobacher: "TYPO3 Extbase: Modern Extension Development for TYPO3 CMS with Extbase & Fluid" (note that current edition is for TYPO3 7. You may want to check if a newer version of the book is available)