There would be a steel plate most likely for the seat pan with nuts welded to it , then the frame would have lugs on the inside of the seat hoop that bolts pass though to hold it on,
the other way would be to weld the bolts to the seat pan and have the nuts underneath.
As for the seat having sharp edges , thats obtained by using high density foam to get it looking sharp.
,
Also making it attach perfectly is all about taking the time to make the seat pan line up right when you are fabricating it in the first place