There are a couple of ways to go about it. A common way is open party-list proportional representation. Each party puts forward a list of candidates. The number of elected candidates depends on the proportion of votes with respect to the total (e.g. 10% of votes with 100 total seats means 10 seats for that party), where those candidates who received the most votes within the party list get elected.
The % of the vote a party gets is the % of the reps they get.
And the party chooses the reps, or how is that done?
There are a couple of ways to go about it. A common way is open party-list proportional representation. Each party puts forward a list of candidates. The number of elected candidates depends on the proportion of votes with respect to the total (e.g. 10% of votes with 100 total seats means 10 seats for that party), where those candidates who received the most votes within the party list get elected.