Learn Hardcore Korean: jeoneun gukjeseon jwaseokboda guknaeseon jwaseogi deo johasseoyo. - I liked domestic flight seats more than international flight seats.

Because in the comparison you’re stating what is “more” (더 좋았어요). That more-liked item is the subject of 좋다, so you use the subject marker 이/가. Since 국내선 좌석 ends in a consonant, it takes 이.

Can we replace 이/가 with 은/는 after 국내선 좌석?

Technically yes, but:

이/가 here clearly marks the item as the grammatical subject of 더 좋다 in a comparison.
은/는 would make 국내선 좌석 the topic, which can shift the nuance or sound less natural in this specific comparative structure.

Why is 더 placed before 좋았어요, and what does it mean?

더 means “more.” In Korean comparatives, you put 더 directly before the adjective or descriptive verb to express the comparative degree. So 더 좋았어요 literally means “was more good,” i.e. “liked it more.”

Why is 좋았어요 in the past tense? Could I say 더 좋아요 instead?

좋았어요 (past tense) implies you’re talking about a completed experience—e.g. you already flew and sat in both.
If you’re stating a general or current preference (not tied to a past event), you can say 더 좋아요 (present tense):
- 국제선 좌석보다 국내선 좌석이 더 좋아요.

What do 국제선 and 국내선 mean?

국제선: “international (flight/route)”
국내선: “domestic (flight/route)”
When you add 좌석 (“seat”) you get “international flight seat” vs. “domestic flight seat.”

Is it okay to drop 저는 at the beginning?

Yes.

저는 is the topic marker (“as for me…”).
In casual or context-clear speech, you can simply say:
- 국제선 좌석보다 국내선 좌석이 더 좋았어요.
  and it’s understood you’re talking about your own preference.