Let me try this again.
There are trace amount of toxic substances in the environment. This has always been the case because the real world is messy. In response, creatures have evolved various ways of dealing with these things and in general, in their natural environment, critters do an OK job of this. This is why all that “detox” and “cleanses” from the naturopathic/New Age/woo-woo types are unnecessary - you have a liver and kidneys and various enzymes that perform the detoxing your body needs. A couple billion years of evolution have worked out a lot of this, as opposed to whatever new fad is making the round on social media.
Now, all critters that eat also, inadvertently, consume some unneeded or unpleasant stuff from the environment. Some of these things can accumulate to some degree in the body, but usually not to toxic levels. However, as you go up the food chain these things become more and more concentrated. In some cases, the animal has even evolved to make use of these things: as an example, monarch butterflies don’t produce the poison/toxin that make them taste horribly awful to potential predators, even to the point of inducing vomiting in birds. They absorb this toxin from the milkweed leaves they eat as larva. Instead of being sickened, they purposely accumulate this for their own needs. Some species of poison arrow frog and puffer fish do likewise, utilizing a toxin in their diet as a defense. In other instances - mercury or arsenic, as examples - the stuff is just bad for life as we know it. Fortunately, mercury and arsenic are not common in most earthly environments. (Mostly - there are exceptions but we’ll skip that for a bit). Your body can deal with the tiny amounts found in normal environments (assuming no toxic spills due to civilization and chemistry).
Most humans (vegans aside) eat from the top of the food chain. Meaning anything nasty in our food will become concentrated in our bodies. Whatever Bad Stuff the cows in our area eat - traces of lead in the soil, or traces of arsenic in the water - can now be detected in minute amounts thanks to the wonders of modern science. And slightly greater amounts will be found in the bodies of the people who eat the cows or drink their milk. Again, for the most part, our own bodies can handle this.
The problem is when our bodies can’t handle it. Maybe our water supply becomes contaminated with lead - a single glassful consumed by someone traveling through the area might not be a problem, but people living there and drinking many, many glassfuls, using it in their cooking, in their coffeemaker… You might wind up in a situation where the quantity of the Bad Thing entering the body exceeds the ability of the body to eliminate it before symptoms appear. This is what happens with heavy metal poisoning. Yes, there are medications that can help your body eliminate this stuff at the maximum possible rate, but it still might take awhile to get the toxin down to safe level again.
Then there are “forever chemicals” that can get into the food chain and, well, they don’t break down over time, or with exposure to sunlight, or whatever, and don’t really go away. They’re everywhere now, and while in minute amounts they don’t have an effect (just like the minute amounts of, say, uranium found in Himalayan Pink Salt isn’t enough to cause problems if you eat reasonable amounts) they can be concentrated in the food chain just like anything else.
So, for most people none of this is a problem, but sometimes a person gets a big dose/exposure. In those cases yes, they need to be “detoxed” but that’s a medical procedure and not something you’d want to do at home with juicing or fasting. And, because it may take a long time to get rid of these things you can wind up with long-term and/or chronic effects.
An example of this is Chloracne, and there’s really no treatment other than treating/relieving the symptoms while the body very slowly tries to eliminate the toxin. But few people encounter this problem. Mercury poisoning is another bad one, and one that is possible to achieve via diet if you eat too much from ocean predators (whale and dolphin meat are apparently very high risk, but people have achieved mercury poisoning from eating too much contaminated tuna, which is why mercury levels in sea food are a Big Deal). But, again, mercury poisoning is something that requires actual medical attention, not fasts or juice cleanses or whatever.
And finally - some “actual toxins” tend to be stored in fat. In such instances, symptoms might not appear until the person starts to lose weight due to the body breaking down fat stores for energy. In such cases, a “cleansing fast” could induce symptoms rather that “cleansing” a person or improving their health. In such cases weight lose needs to be gradual so as to avoid greater amounts of Bad Stuff in to the blood stream than the body can handle. Engaging in fad diets/fasts can actually cause the problem to worsen in such cases.
Does that help?