Firstly, there are 5 compounds to choose from. They call them C1 to C5, hardest to softest. Pirelli decides which 3 to bring to a race weekend depending on the circuit characteristics (abrasive or smooth) and the location temperature/climate. Typically they bring 3 consecutive compounds in order (like this week they have C1, 2, and 3), but sometimes they skip one and may do something like C1, C2, and C4. Regardless, these are always marked with corresponding colors, red is softest tire for the weekend, yellow is medium, white is hardest. They also being intermediate and full wet tires (compounds of these are always the same and do not change) just in case. Teams get 13 sets of tires for a weekend. Pirelli will specify two sets that must be saved for the race (typically one set each of the two harder compounds), and one set of the softest compound is saved for qualifying. For the other 10, the teams used to be able to choose what they wanted, but last year and this year Pirelli has made that decision for easier logistics during pandemic times. So, for example, each car may have something like 8 soft, 3 medium, and 2 hard sets for practice, qualifying, and race. Teams then give back two (used) sets to Pirelli after FP1, 2 after FP2, and 2 after FP3. Pirelli will study these used tires to see how they wore to help gather data for the future. That leaves 7 sets left for Q and the race (including the three mandatory sets as described above). So, taking the two mandatory race sets away, teams will typically want to save another new set of tires for the race as well, leaving 4 sets for Q. For the top teams, that usually means using 1 in Q1, 1 in Q2, and 2 in Q3. However, if you make it to Q3, you actually get one more set of soft tires to use (a 14th set). Sometimes teams will make a run on used tires in some of the Q sessions as well, or, if they didn’t get a clean run in, may have to use another one of their new soft sets which means they may no longer have that extra new set for the race (but can still use used tires for it). So in the end, the teams usually end up with 3 new sets (and 4 used sets) for the race, but they will always have 2 at minimum. For cars that made it to Q3, they are required to start on the tires that their Q2 time was set with. This is why you see some teams try to use a harder medium compound to make it into Q3 with, since starting the race on a harder tire opens up more strategy options. Everyone else behind the top 10 on the grid can choose whichever tire of the sets they have left to start the race on. Sorry that was long, but I think I got most of it. There’s some other rules that come in to play when it’s wet, like being able to trade some of your dry sets for intermediate or wet sets, but I don’t remember them exactly.