I can understand the meaning of the red underline part in the picture, however, I can't tell the part of the sentence of those infinitives in the subordinate clause. Is it the actual subject of the subordinate clause and 'it' is a dummy subject?
2 Answers
It means she was happy to have Mariam as a target to focus her anger and grief on.
Simplifying the structure,
She now had a target to focus her anger on.
The sentence would be better as
to have a target on whom to focus all her shimmering anger, her grief.
That would be better because the word "focus" is usually used with the preposition "on", rather than "at", and "whom" works better for a target that is a person (Mariam).
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Laila was still shocked at how easily she'd come unhinged, but, the truth was, part of her had liked it, had liked how it felt to scream at Mariam, to curse her, to have a target at which to focus all her simmering anger, her grief.
Yes. This is a type of cleft clause with a dummy "it" and an infinitive phrase (actually, three phrases) acting as postcedent. Without the cleft structure, a gerund phrase is the more natural choice for the subject:
She had liked how it felt to scream at Mariam.
She had liked how screaming at Mariam felt.
The original sentence has an asyndetic coordination of three infinitive phrases, complicated by a relative clause within the final infinitive phrase and another asyndeton as the direct object within that -- quite a long and involved subject for that content clause. It's more natural to give the simpler content clause a dummy subject before delving into the extensive depths of the subject's referent. An equivalent coordination of gerund phrases in canonical position is harder to parse:
She had liked how screaming at Mariam, cursing at her, having her as the target at which to focus all her simmering anger, her grief, felt.
By the time readers reach that terminal "felt", they may have forgotten the reason to expect a verb there.