I heard that symbicort (budesonide and formoterol) contains formetrol which can be used as a prn because it have rapid action and long duration b2 agonist?

No! Symbicort (budesonide and formoterol) does contain formoterol but this is considered a long acting beta-2 adrenergic agonist (laba). It is not rapid acting and should not be used in an emergency situation. As such, symbicort (budesonide and formoterol) should not be used on an "as needed" or prn basis.
Formoterol. Formoterol is a long-acting ?2 agonist that has an extended duration of action (up to 12 hours) compared to short-acting ?2 agonists such as albuterol, which are effective for 4–6 hours. Formoterol is used as "symptom controller" to supplement corticosteroid therapy (e.g., fluticasone). It should not be used for resuce. A "reliever" short-acting ?2 agonist as albuterol is still required, .