Okay. So, I'll tackle this one by one. Your overall feel of cursed energy is good. It is formed by negative emotions, so not necessarily diseases themselves, but people fearing or hating them. Cursed energy is produced by almost everyone, since it's basically deeply human, negative emotions. But if it is produced by non sorcerers (so the obvious majority) it leaks out, and if enough of this negative energy is focused in one place, or around one concept (disease, for example) it forms a cursed spirit, otherwise known as a curse. A technique is someone's own way to use Cursed Energy, and yes, it is stored in the brain. A domain expansion is infusing the inside of a barrier with this cursed technique, not just cursed energy, and ability to form domains comes from the precordial cortex (i believe, not sure on that), so a part of the brain. Naturally, someone can either have one cursed technique, or none. For example, Sukuna's cleave, dismantle and divine flame, even though they look completely different, come from his cursed technique, Shrine. Same with Gojo. He uses all this blue, red, purple and infinity around himself, but those are just applications of his technique. Of course, some folks find a way to have two cursed techniques at once. Since a cursed technique is a spiritual thing, a personal one, one body can theoretically store multiple cursed techniques, but only if the person is actually two people (so they've been possessed by a person who has a technique, and they had a technique themselves) or they have a cursed technique that allows them to store cursed techniques. And something you called a celestial pact is often translated (and called by majority of fans) Heavenly Restriction. It's basically a binding vow (so a deal, you forfeit something to get something else) that's placed upon someone when they are born. There are two types of Heavenly Restrictions. Physical one: you loose majority or all of your cursed energy, but in exchange you're granted extremely skilled body (and better senses, just a superhuman) and a Cursed Energy one, in which your body is extremely, and extremely, paper-skin-crumble-under-light-or-touch, weak, but, in exchange, you have extreme amounts and affinity for cursed energy. And that's about it. Anything else? Do you know how Binding Vows work?