In Mexico elections are based on a single round of voting. With a plurality sufficient to win.
Parties register under Federal law (subject to certain requirements, such as achieving 3% of the vote or passing other tests of a certain minimum level of support) Then the selection of a party candidate is subject to internal party processes.
As an example, López Obrador was selected at the MORENA national convention in 2018. There is no system for wider involvement in the selection of candidates, as in the (almost unique) primary system in the USA. Nor is there a run-off system, as used (for example) in France.