My guess is that in most of those systems, adding a Skill or an EmploymentHistory for an Applicant doesn't require adding a column to the Applicant table. Instead it just requires adding a row to the Skill or EmploymentHistory table.
If your design requires columns in the Applicant table for various Skills, then your Applicant table isn't properly normalized. Have a look at the Wikipedia entry for
Database normalization, where you can learn about database design techniques which avoid your problem. (I was going to call them "modern" database design techniques but the principles were all documented back in the 1970's so "modern" is a really bad
word to describe them.)
It's also possible that those systems you refer to are using a NoSQL type of database, but similar principles apply in that case too.