Learn Hardcore Korean: geumyoil ohueneun jeojjok geonmul oksangeseo swigido haeyo. - On Friday afternoon, I also rest on the roof of that building.

금요일 오후에 means “on Friday afternoon.” Adding 는 (→ 금요일 오후에는) topicalizes that time frame—“as for Friday afternoons…”—and is commonly used to talk about habits or general statements linked to that time.
Example:

주말에는 운동을 해요. (On weekends, I (we) exercise.)

What does 저쪽 mean and how is it different from 저기 or 거기?

저쪽 literally means “that side” or “over there” (a direction farther from both speaker and listener).

저기 = “there” (a neutral “there”)
저쪽 = “that way/side over there” (more directional)
거기 = “there” closer to the listener’s side
Example:
저쪽 건물 = “the building over there (in that direction)”

Why is 옥상에서 using 에서 instead of 에?

에서 marks the place where an action occurs. Since 쉬다 (to rest) is an action, you say 옥상에서 쉬다 (“rest at the rooftop”).

Action verbs → use 에서
Existence or destination → use 에
Examples:
교실에서 공부해요. (I study in the classroom.)
집에 있어요. (I’m at home.)

What does -기도 해요 indicate in 쉬기도 해요?

The pattern -기도 하다 (attached to a verb stem: 쉬다 → 쉬기도 하다) means “also do …” or “sometimes do …,” suggesting it’s one of several activities.
In 금요일 오후에는 … 쉬기도 해요, it means:

“On Friday afternoons, we also rest there,” or
“Sometimes we rest there.”
Example:
주말에는 친구를 만나기도 해요. (On weekends, I sometimes meet friends.)

How does 쉬기도 해요 differ from simply 쉬어요?

쉬어요 = “I rest.” (a straightforward statement)
쉬기도 해요 = “I sometimes/also rest.” (implies variation or that resting is one among other actions)

What exactly is 옥상, and how is it different from 지붕?

옥상 = the flat rooftop area you can walk on or use (a roof terrace).
지붕 = the roof structure itself (the sloped covering protecting a building).
Example:
옥상에 의자를 놓고 쉬었어요. (I put a chair on the rooftop and rested.)
지붕 위에 눈이 많이 쌓였어요. (A lot of snow piled up on the roof structure.)

Why is there no subject in this sentence? Who is doing the resting?

Korean often omits the subject when it’s clear from context. 쉬기도 해요 could mean “I,” “we,” “they,” or even “you” politely, depending on the situation. You infer the subject—most likely “I” if it’s your routine, or “we”/“they” if talking about a group—based on surrounding context.