"How is chi used in Buddhist practice?"
Buddha ignored the fascination people had with all this and classified it as "the five elements" knowing that even the slightest attraction to these low physical phenomena would waste people's time and detract from the many more worthwhile goals. You don't have to know anything about chi in Buddhism because in Buddhism the method of true awareness is enough to dissolve all blockages and problems that might come up.
This is why so many uneducated Buddhist monks who didn't know anything about any of the other traditions still won the prize of Enlightenment! One of them was the patriarch of the Zen school, whom other jealous disciples tried to hunt down..
In those times, monks would be all too interested in those things, at the expense of understanding one's True Mind, which is beyond body and mind... If Buddha even mentioned the word chi, there would be a million and one distracting questions popping into people's head at those times--that would probably derail people's focus entirely... which is why Gautama supposedly returned as Padmasambhava and created the Tibetan school--much much later after the heart of Buddhism was established. (Tibetan school by the way is still aligned with Buddhism but has an integrated focus on the wisdom-energies.)
Nonetheless, chi is related to different karmic habits that we have and once these energies are dissolved the personality characteristic is also transformed--and vice versa.
Buddhism aims at a much higher target than physical perfection, beyond even physical immortality, and this is why it is possible in Buddhism to go beyond the body to transform the mind at its root, attaining things that are definitely Buddhist ("non-common supermundane achievements") and other things that are beyond form but common to all spiritual schools ("common supermundane achievements").
Buddhism aims at the highest possible level: transforming the 8th consciousness, the storehouse of karmas from our endless lives... becoming a radiant actualized Buddha helping all beings win Awakening, becoming free from cycles like birth-and-death and suffering. To Buddhism, these super-powers are nothing but wasting time developing tricks, totally distracted from our human and spiritual problem. That is, unless it is for skillful means (see the parable of the burning house).
With that Buddhist warning aside, to answer your first question, there are many different types of chi, based on various organs. This falls within the realm of TCM and there are many uses to knowing such things, especially in diagnosing one's body. I would love to have an intuitive TCM doctor analyze my sitting posture--lots of constructive advice can be gained from that!
So what is chi in related to Buddhist practice and meditation? How can we make use of such learnings to motivate our Buddhist practice?
The Taoists have mapped out a timeline for what the excellent cultivators and Buddhists go through:
jing > chi > shen > emptiness >> Tao
When one practices Buddhism 100% correctly (celibacy + samatha-powered vipassana), this process happens automatically all the way to "seeing the Tao"/bodhisattvahood.
Due to one's celibacy, one has stored up jing (semen) which after 3 months of jhana eventually starts to transform into chi. One will feel less hungry and can fast for a long time. One is qualified to do the things posted in the picture. Eventually after at least a year like this, one's chi will transform to shen. For several years this process will continue, one's sleep needs will drastically reduce. After even more refined letting go, one will be at the stage of "emptiness" free from dualities and one step away from turning around and "seeing the Tao"
Also, the Taoists have fancier, more interesting names than this - I'm just summarizing.
I recommend the dense book "How to Measure and Deepen Your Spiritual Realization" for a complete understanding of this subject of energetics and where your concern on energetics absolutely must stop in order to progress and not subconsciously focus on lower things and hold yourself back from awakening the formless, nondual prajna wisdom.
Whether one is a monastic or a layman, in this day and age there is a lot of confusing bioenergetics fascination so it is important to clear up this matter for yourself because even if I say "just study Buddhism, and keep noting, all that fancy stuff is nothing they will lose it after they die"... you are still curious and vulnerable to false information--especially since you want to be able to do cool things like be free of sexual desire (when your jing is full), no longer need to ingest food (when chi is full), and no longer need to sleep (when shen is full)... at the best false information + fixation might lead to nonbeneficial/retarding practices and at the worst, a painful fiery death (I refer you to the Buddhist story of Hakuin).
2nd question: yes, it is a result of staying in that state. After achieving the 1st jhana on a mental level, one must reside in this state until the body completes transforming. After one's body and mind are in sync (this may take a few months if one achieved jhana/insight quickly), one's state will stabilize (and one can go to even higher states more easily)... and one can feel and train one's chi to do various tricks--beneficial and harmful. One can also ignore those lateral pathways and continue on the Buddhist journey.
Go here for more nondenominational info and definitions.
Also read "Master Huai-Chin Nan - Internal Martial Arts Nei-Gong" to have all your chi-related questions answered, especially related to Buddhist practice.
The dense book I mentioned earlier has excellent case studies as well on how chi manifested in different practitioners' lives.