It does not, and that is the problem
How Does a Pill Know Where to Go?
A swallowed pill goes everywhere in your body. It works because it only fits one thing, not because it knows where to aim.
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Step 1 of 5
The blood takes it everywhere.
A dissolved medicine is absorbed through the gut wall and joins the blood, which reaches every part of you within about a minute. Nothing steers it.
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Step 2 of 5
It works where its shape fits.
Cells carry receptors, which are sockets of a particular shape. A drug molecule shaped to fit one of them changes what that cell does and passes everything else by.
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Step 3 of 5
Side effects are the same shape somewhere else.
If that socket also exists somewhere you did not intend, the drug acts there too. Most side effects are the medicine working correctly in the wrong place.
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Step 4 of 5
The dose is a balance, not a maximum.
Too little does nothing and too much reaches sockets that should have been left alone. The gap between those two is what a dose on a box represents.
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Step 5 of 5
The body starts removing it immediately.
The liver breaks medicines down and the kidneys flush them out from the moment they arrive. That is why doses repeat: the level falls and has to be topped up.
Which is why a coated tablet exists at all. The coating is chosen to survive the stomach's acid and dissolve further along, so the drug is released where it can be absorbed.
The short version
A pill spreads through the whole body and acts only where its shape fits a receptor, which is also exactly why side effects happen.
Try it yourself
Look at a dose on a medicine box. It names an amount and a spacing, and both of them matter.