First NEVER attach torque converter to flexplate first, NEVER EVER!!!
(flywheel is for manual/clutch, flexplate is used on automatic)
Yes, the torque converter goes on to transmission before installing in vehicle, it runs the front pump(main pump) in an automatic transmission.
So you put the torque converter on to the input shaft and push it in towards transmission, after it stops, rotate torque converter while pushing in, not hard, you will feel it side in a bit more each time it latches on to front pump tabs, sometimes it will slide in 2 or 3 times.
Continue rotating until it won't slide in any more.
When you install the transmission it should bolt up to the engine and be flush, you may need to wiggle it a bit but never use the bolts to force it into place, something is wrong if you feel you need to do that.
The torque converter will then slide forward a 1/2" or so to bolt up to the flexplate.
You have to rotate the crankshaft/flexplate to install all the torque converter bolts, so remove spark plugs from engine to make that easier.
Get all the bolts in but not tight first, then tighten them.
My first automatic install went sideways on me, I did put torque converter on flexplate and then installed trans, BIG OOPS, had to force trans in and used the bolts to snug it up.
Got it all back together, started it up, put it in gear and nothing, WTF!
No forward, no reverse???
I had broken the front pump by forcing torque converter into trans, no pump, no pressure, no go!
And it requires a complete rebuild to replace the pump, expensive lesson.