Many belts these days are fitted with rather effective, lightweight, snap-together plastic buckles. The female end of these is almost always permanently stitched to the belt.
This means that, if you need to cope eg with an increase in waist size, you really only have the option of buying a whole second belt.
Today’s invention (in orange) is a secondary male-to female insert which can be used to extend one’s belt (by using several of these, if necessary).
Make these small and/or flexible enough and the whole belt might be made of them.
A similar approach could be used eg for watch straps.