Is it legal to open the back door on the first floor of residential quarters?

Legal analysis: it belongs to infringement and damages public property. You can talk things over with the property. If the property is unattended, you can continue to report it upwards.

Legal basis: Article 275 of the Criminal Law of People's Republic of China (PRC) intentionally destroys public or private property, and if the amount is relatively large or there are other serious circumstances, it shall be sentenced to fixed-term imprisonment of not more than three years, criminal detention or a fine; If the amount is huge or there are other particularly serious circumstances, he shall be sentenced to fixed-term imprisonment of not less than three years but not more than seven years.