Still no idea how this Nations League works
To be honest, the Nations league is one of the few modifications of late that UEFA has gotten correct. Essentially, they've scrapped the European int'l friendlies and put the Nation's league in place.
You got the A, B, C and D leagues with the top teams in the A league. The winners of each group in league A go on to play in the Nation's league SFs and Final. The bottom team in each group in league A gets relegated to league B. The winners of each group in League B get promoted to League A and the same pattern applies for C and D, although D has fewer groups so there's like a relegation play-off for League C or something like that.
The big kicker for Nation's league is there are slots awarded to the best performing teams for UEFA Euro and WCQ that didn't automatically qualify for the tournaments.
So let's say Italy goes on to win their group and the Nations league but fails to qualify outright for Euro 2024. Since they won their group, they'll have a slot to try and qualify through play-offs.
For the 2022 WCQ, Austria and Czech Republic made the WCQ playoff rounds by virtue of winning their Nations league group and being the highest ranked teams that didn't automatically qualify for the WC. Essentially acts like a get out of jail card for some of the bigger nations
I actually think it's a very nice setup since there's something to at least play for and the nations can win a trophy as well
Quick video explanation.