Is this a yearly event and I just googled it, it was played against Netherlands. Were France, Belgium and Spain not invited?
Every two years is the plan, this was the first one. All European teams were in it.
It's largely replaced friendly international matches in the seasons where there are no qualifiers for either World Cup or Euro tournaments, having competitive football instead. There is also a path to qualify for Euro 2020 through this for teams who end up not qualifying the traditional way.
In the top league (League A), the best 12 teams according to rankings were divided into 4 groups of 3. Each team played each other twice during the season. The winner went to the semi finals, 2nd in the group gets to stay in League A the next time they have this in 2020/21, 3rd got relegated and will be replaced by one of the winners from League B for next time.
The groups finished:
1st Netherlands, 2nd France, 3rd Germany
1st Switzerland, 2nd Belgium, 3rd Iceland
1st Portugal, 2nd Italy, 3rd Poland
1st England, 2nd Spain, 3rd Croatia
Netherlands beat England, Portugal beat Switzerland in the semi finals played after the end of the season, and you know the result of the final already.
https://en.wikipedia.org/wiki/2018–19_UEFA_Nations_League