Why "it" instead of "them" in "make it into"?

In this report news, there is a sentence about ports:

Seven Chinese ports made it into the top 10 ports in the world in terms of container throughput in 2018.

Why "it" is used? Should we use "them" because "ports" is plural?

Solution 1:

To make it is an idiomatic expression meaning, as Macmillan has it,

to succeed in a particular activity

The "it" does not refer to the subject, but to the fact of some achievement, milestone, or cutoff being met. This may be compared with other uses of make meaning to attain an objective, like making a deadline or making the team. According to the OED, these uses are derived from the nautical sense of make it meaning to reach a certain place or cover an intended distance successfully.