In the absence of food your body looks internally at what it already has (stored energy or weaker cells/protein that we use for fuel), alongside a host of physiological responses that allow you to perform at your best.
The reason these responses occur is because we're evolutionarily designed to sharpen up when food isn't available - for example, to hunt. If we were unable to perform in the absence of food we wouldn't have lasted very long.
Some of these responses include: activation of the sympathetic nervous system, plus the release of adrenalin and noradrenalin, cortisol, and human growth hormone.
The benefits of fasting include: facilitates fat loss, lower insulin levels and greater insulin sensitivity, reduced oxidative stress, reduced inflammation, improved cognition and brain function, mitigates the fall in metabolism from fat loss, improved digestion, greater diversity of gut microbes, as well as lifestyle factors such as increased productivity, reduced decision fatigue, and reduced time preparing food (and everything that comes it).
The most common fasting protocol is 16:8, meaning you consume all of your calories in a 8 hour window of the day. The smallest window I suggest is a four hour eating window (20:4), with my personal favourite being somewhere in between (18:6). The Cresendo method is a less intense form of fasting, 12 hours of fasting and 12 hours of feeding.
The best protocol for you is the one you can sustain and enjoy. If you spend hours of the day with your mind closer to the fridge than on what you're doing - this is not healthy nor sustainable. Find a practice that elevates and improves your life, not one that hinders it. You have nothing to prove and fasting is not a cult with strict rules or regulations - the key message is not eating around the clock.
As a whole, only consume water during your fast, if you're fasting specifically for gut rest the less you consume outside of water the better. If gut rest isn't your primary goal, tea or coffee can be consumed in moderation - remembering that you're on an empty stomach so excessive coffee or caffeine can cause irritation to the gut lining.
Train when it suits you best. Training performance and body composition is impacted very little whether you train fasted or fed, just ensure that you're not over-training in a fasted state as it's highly taxing on the body. For optimal performance and recovery train fed. If training well before you break your fast you can continue fasting but I consider this an advanced technique.
Fasting is another tool in the armoury to be used as and when it suits your lifestyle, don't be ruled by the numbers, listen to your mind and body and find your own fasted lifestyle.
If you want a complete guide to fasting I spent two years collecting the existing state of knowledge on the topic in my book "The Fasted Lifestyle" (linked below).