封 and 栓: a sealed closure and a plug for an opening
Compare an envelope's sealed closure with a stopper in a bottle or hole. Read the objects of 封を切る, 封をする, and 栓を抜く.
Open the sealed closure
封, read ふう, refers to a seal or sealed closure. 手紙の封を切ると、中から古い写真が出てきた。 says that an old photograph came out when the letter's seal was broken.
封を切る opens the sealed closure. The photograph coming out is also stated; this is not a report of removing a stopper or necessarily of the photograph falling.
Remove the object plugging an opening
栓, read せん, means stopper or plug. 瓶の栓を抜くと甘い香りが広がった。 says that a sweet fragrance spread when the bottle's stopper was removed.
抜く acts on the stopper closing the bottle. Read what was closed without making an envelope's seal and a bottle's stopper identical physical objects.
Identify what is sealed or plugged
大切な書類を入れた封筒にしっかり封をした。 says that the envelope containing important documents was firmly sealed. 封をする is the sealing action.
水槽の底にある穴をゴムの栓で塞いだ。 says that the hole in the bottom of the tank was plugged with a rubber stopper. Distinguish the sealed closure from the object plugging a hole.
Is 栓 in 瓶の栓を抜く the same object as an envelope's sealed closure?
Show the answer and why
It is the bottle's stopper. The comparison concerns a sealed closure versus an object plugging an opening.