Sometimes it happens by accident. People miscalculate the mature size of their plants when they're small.

Sometimes it's a matter of luck I think, like when I planted a dark sweet potato vine that wandered around among several other plants and tied them all together. Sometimes I actually do it with some 'malice aforethought', knowing that this cute little thing will spread out and make a larger plant, and then put bulbs in where they'll have room to spread, and it doesn't take very many things like some salvias to fill space fast.

I think the lush look can be very nice but is often only temporary. It seems to be a product of serendipity. Enjoy it while it lasts but after some time passes, the owner is often left with a nightmare to maintain. If you do strive for this, it is best to "over-plant" with plants that are easy to pull up IMO, some have been mentioned above.

I prefer the zen look that actually allows bare mulch to show thru in spots. Seems to add an element of tranquility that the "lush" gardens lack IMO. Though I also like to view the "lush" gardens when they are at their peak, I just don't want to think about the maintenance when they start losing their luster. Each to his own.

If you have weeds you don't have enough plants. I often dig a small hole & stuff plants in spots. If a weed can grow there so can a plant. Then if they get out of hand I just dig out what I don't want.
Tally HO!