が真実で

is true

→ が
phrase Uncommon #16,900
is true This is a sentence fragment: が marks the subject, and で connects 真実 (truth) to what follows. It often appears in forms such as が真実である.
JA: 彼の話が真実であると信じています。
EN: I believe his story is true.

すべてが真実で輝いて見えるこれを表現したいだがどうすればいい?

How can I express the idea that everything looks true and radiant?

— 8 1/2 (1963)

どちらが真実で虚構なのか?

Which is true and which is fiction?

— 落下の解剖学 (2023)

何が真実で虚構なのか?

What is real and what is fiction?

— 落下の解剖学 (2023)

彼が言ったことが真実であるはずがない。

What he said can't possibly be true.