Long story short. Attribute assignment is one of the most disputed topics in all of D&D. There are three primary models to work from.
1) Rolling. 4D6, dropping the lowest roll. Repeat this six times. Assign each total to the state of your choice (there is a lot of variation here in terms of how many dice to roll as well as what type of dice to roll, but this is the official way in 5e). This model is, to my surprise, the actual default method of character generation in 5th edition. After a half dozen 5e campaigns, I went back to review character generation while I was considering new ways to run character generation and was shocked to see that rolling was the default, because I assumed it was one of the following methods.
2) Point buy. The system set up in Baldur's Gate 3 uses this and it is exactly translated from the 5th edition rules. This is the most popular way of making characters in 5e.
3) Standard array. Take the following numbers: 15, 14, 13, 12, 10, 8, and assign them in any order you wish to your six stats. Then apply your racial or template benefits on top. This is meant to be a way to quickly generate balanced characters, and is a very useful tool in maintaining balance in 5th edition. I actually think this method is best as both a player and a DM. As a player, I find that it helps to prevent me from over-optimizing my characters and results in more interesting choices as a result.
There is that 4th rare option of 1D20 re-rolling anything 3 or less or for the super hardcore - no rerolls.
