Search for "VAT MOSS", should be easy to find good resources.
Some countries also have requirements as to invoice language, or conversion rates that you need to use for foreign currencies. In my case (Poland), I can use the European Central Bank (ECB) rates for VAT, but not for income tax further down the line, so I have to integrated with two central banks to get their rates.
Overall, there is lots of overhead. There is a disconnect between EU officials and small modern companies. Officials want to "promote innovation and entrepreneurship" while introducing heavy-handed legislation (really meant to hurt Amazon, in case of VAT MOSS).