The remaster itself is free for everyone who has a copy of the original (aside from those who got the game through PlayStation Plus), but Episode Intermission has to be purchased separately for $19.99, or as part of Intergrade if you don't qualify for a free version.

Do I have to buy FF7 INTERmission?

About FF7 Remake Intergrade and EPISODE INTERmission DLC

The EPISODE INTERmission DLC with Yuffie Kisaragi is a standalone episode, which will need to be purchased separately if you upgrade a previously purchased PS4 version of FF7 Remake to Intergrade on the PS5.

How do you get INTERmission in Final Fantasy 7?

From the main menu of Final Fantasy VII Remake Intergrade, you can hit R2 to go to a separate INTERmission menu.