Google is your friend and you'll find easy explainers on CalDAV and CardDAV.

But as yoy've pointed out as a beginner you're trying to reinvent the wheel. While it's probably quite possible to extract data from a spreadsheet this will probably be a bit more complex as you'll need to use regular expressions to extranct the data and use a CSV formatted spreadsheet. Most PC's and Phones have contacts apps to manage your contacts and calendar apps that can generate birthday calendars from info from your contacts. You'll make your life easier by using those apps. Nextcloud allows you to store that data in a central location and sync it to your devices by using the protocols mentioned.