The old standard was that a front fork ended just above the top tube of the frame. Then a L-shaped bit called a (quill) stem was inserted into the fork and locked in place with an expander, and at the other end of the the stem the handlebars were clamped in place. Aheadset refers to that the fork no longer end immediately above the top tube, but continues upward a little bit. And the stem isn't inserted into the fork anymore, but it's a straight piece where one end of the stem clamps around the top of the fork and the other end clamps around the handlebars.
A BMX stem is what connects the handlebars and forks to the frame.
i think most stems fit but i would take it to the place you are buying it or take the messurements of the old 1
no bmx bikes hndelbars are 22.2 mm and mtb dirt jump are 25.4 mm. the dmr swarm stem is ment for mtb dirt jump bikes (25.4)
Tighten the stem
Anything from USD 20 to USD 200
The forks go through the head tube and headset, the stem anchors the fork and then the bars attach to the stem, so the answer is the fork and stem.
the fork gyro (if have) and stem and mostly important the headset
polished
It´s the Stem, it holds the bars and match with the frame with the help of a headset.
Depends on what type of stem/for it has. I'll add a few links below so that you can read up on it.
BMX
you spell it BMX