Why is it "svoji" and not "svou"?
both are fine
Why is “Myslím na svoji holku” incorrect?
Just an omission. Fixed.
Ah, okay - thank you for the quick follow-up! I generally default to assuming that it’s a misunderstanding on my part when it comes to producing Czech sentences, which is why I didn’t report.
why "moje" is wrong?
You need accusative "moji" or "mou".
Does svoji not mean "your"
svůj relates to the subject of the sentence. It can by my, your, his, her, our, their...