What qualities and achievements should be mentioned in a letter of recommendation?

When I was writing a letter of recommendation for my friend, I was also unclear about the best qualities to highlight. But I really wanted to help my friend, so I started looking for advice on the Internet. On one of the forums, some guy recommended me an article about reference letter for a friend for a job at https://jobssite.ca/advice-on-writing-personal-reference-letter-from-a-friend/ . It says how to choose the right qualities and achievements and describe them favorably. It also emphasizes that you should focus on professional skills, personal qualities and joint projects. Thanks to this, I was able to write a convincing letter and my friend was satisfied. I recommend reading this article!

What is the difference between a formal and informal letter of recommendation for a friend? Should an informal letter follow a strict structure?

I want to write a letter of recommendation for my friend, but I’m not sure which qualities and accomplishments to highlight. How can I best choose what to focus on — professional skills, personal qualities, or joint projects we’ve done together? What examples from our interactions might best illustrate his strengths and make the letter more memorable?