"How does PR/AV work in countries that have PR/AV?"
In Germany, the system is basically the same as in Scotland for the Scottish parliament elections. It's called the additional member system.
How it works is that you get two votes. One vote is for your particular constituency and then one vote is for the party that you want to govern.
The vote for the party that they want to govern is the more important one as that decides the final number of seats allocated to each party. Although sometimes people do split their vote, for example if they really like their constituency MSP but don't like their party.
In total there are 129 MSPs in the Scottish parliament. Some of these are elected in constituency and some are appointed by their party.
Who ever wins on a FPTP basis in each constituency becomes the MSP for that constituency. In Scotland there are 73 constituencies and so 73 constituency MSPs
The second vote is then to decide which particular party you want to vote for to actually run the country and it is this vote that decides who the extra MSPs are going to be.
What happens is that they work out how many MSPs of each party there should be according to the party vote (it's called the Regional Vote in Scotland) and then award places to candidates from each party in addition to their constituency MSPs to make up the numbers to the percentage they got in the Regional Vote.
It's probably easiest to explain with an example. This is from the last Scottish election.
The Conservatives won, I believe, five constituency seats (which is 6.8% of the constituency seats) but in the Regional Vote they got 23.5% of the votes and so ended up being given another 26 seats to give them 24% of the total number of seats.
The Green Party didn't win a single constituency seat at all, but in the Regional Vote they got 8.1% of the votes and so ended up being given 8 seats or 6.2% of the total seats without winning a single constituency vote.
The Greens were then in a coalition with the SNP and they are largely responsible for trying to push through some of the more loony things that have come out of the Scottish Parliament over the last few years.