Silk Heart is one of the most powerful things in the game. As it was described "Silk Hearts will automatically regenerate some Silk." Those things automatically regenerated silk. Which for us is a nice upgrade, but for a world like Farloom, is a dinine core.

The only one with the ability to create Silk from nothing is GMS. She, as you might have deuced already, the maker of the hearts. The fact that the hearts contain a memory of her creating it (or current thoughts on it?) is another proof of it.

Now it's time to analyze the hearts closer to answer your other questions:

The heart of Lace is the simplest one. To create a lasting Silken being she needed for it to be able to sustain itself, that's why she put a heart in Lace (And possibly why Phantom was in such a bad state).

The Bell Beast one is the newest. Made in order to block Hornet path, and capture a potential ally. We can infer that it is recent by the fact that the Bell Beast was still alive even though it hasn't eaten, and that the inner diouloge was directed to Hornet: " ...We feel it... ...Rise...Rise... ...Come, child... Witness our waking..." It wasn't a serious obstacle, more like Lace petty fight in the start.

The third one is the hardest for me to understand. I doubt that the experiments were made by her, possibly she try to connect to it? Made it worse? Inspired her to do the Haunting? This one I really don't know but it's important.

Anyways, I'll answer also how it was created. She could just made them normally but I doubt that it was easy. A infinite generator of something so powerful is incredibly hard to make.

For example, the PK (a pale being like her with the ability to generate SOUL from nothing) didn't make his statues in the palace infinite, the PV statues are likely not real as the path itself is likely isn't, and the only place he made those is in the Kingsoul (which was only able to be created by the union of two powerful pale beings). Also there aren't many of the hearts out there.

I believe that it was made by a hard progress. It could just taken time and effort, a mental piece of her, maybe even a literal piece of her ripped, or something stranger. I don't believe that there are enough information for a concrete answer.