The bogie is retained by motor chassis side pieces - there are 3 screws that clamp the metal parts - and have a tiny insulating bush. There are plastic square nuts that fit in recesses- all easy to undo. It the rebuilding that can be fun.
However to replace bogie side frame it is not necessary to dismantle anything - take body off to save scratching it . As you have found the bogie clips on at rear and two lugs at front locate it around the coupler. The replacement is first located on lugs and avoiding mistrapping pickup strips the end clip will with only a little pressure locate around the lug on back of wheelset frame.
The fun part of remantling a split frame chassis is using all three hands to aline bogies, shafts bushes and keep motor spring caps in contact with metal sides and then screwing it together so it runs freely and bogies can rotate and move up and down to accommodate gradient changes.
I end up with couple of sitirrers and some tape to get chassis block up a bit to allow side frames to sit on worktop and use tamiya masking tape to keep back nuts in place. There is a right tightness as too tight and bogies tower binds and so will not rotate and also at a worst case can bend and trap drive shaft.