What it is for
The cover letter answers a question the CV cannot: why this role, at this organisation, now. It is not a prose version of your CV.
Assume it gets thirty seconds. Everything must be legible in that time.
A structure that works
First paragraph: the role you want and the single strongest reason you are a fit, stated concretely.
Second paragraph: one relevant piece of evidence in detail. A specific project, what you did, what changed as a result. One example told properly beats four mentioned.
Third paragraph: why this organisation specifically, based on something real you know about it, and a plain closing line.
What causes readers to stop
"I am writing to express my interest in" is the most common opening and adds nothing. Start with the substance.
Adjective lists without evidence. "Passionate, detail-oriented, results-driven" describes everyone who applied.
Any indication that the letter was written for a different company. Check names before sending.
Using a generator well
Feed in the job description and your genuine experience, and ask for a draft. Then replace every generic claim with something only you could write.
Keep it to one page. If it runs over, the second example is what to cut.