in 6.3 the conversation includes
Estas son las mejores vacaciones de mi vida.
This is the best vacation of my life.
It mentions that vacaciones is treated as plural, which explains the ‘estas son’. Which means the above sentence translates literally to “These are the best vacations of my life”.
If you were trying to specify the difference between
“This is the best vacation” vs “these are the best vacations” how would you do that?