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One concept that you have to understand is that all FAGMANS accounts are linked, there is no difference to the interested agents between your Facebook, and Amazon account, they are all linked to you.
Most social media and other apps, services etc, require your phone number or email. Have you used a different one for each?

The first step is the identification of demographics.
> Pic related
about a campaign Signal led on facbeook and Instagram (facebook owned) where they showed people directed ads based on facebook's cathegorization.
Countrary to what people believe, these ads do not find out information about you.
The ads are created and sent for a specific target. Facebook knows you are a pregnant woman with an interest in fitness, but the advertiser merely sends out an ad to pregnant women who have an interest in fitness.

How this is done is dependent on the site.
Facebook has groups. What those groups are about is reported by the users through tags, in the interest of being found by people who want to be a part of a sports group.
Then, you go and join those groups. Depending on your interest, which is calculated based on your constancy of participation, your interest is ranked.
You have a set of interests derivated likely from group belonging, as well as likes and other mechanisms.
Instagram does this mostly through your likes.
youtube obviously keeps tracks of what videos you watch. These videos have tags.
You can make a weighted accumulation of all tags you've consumed: 
Take the amount of time you watched a video, and assign that as a weight to a tag, then add those tags together and you get: [ "puppies": 3.6, "metal": 2.4, "republican": 1.2, "democrat": -0.6, ... ]
This is known as a profile. These tags are incredibly useful to categorize you. More so if they are weighted. They quite literally are worth their bits in gold.

It is unknown to me how other social media do it, but every single one without exception does this.
There are no arbitrary elements in a social media webpage.

> With smartphones
the issue is darker, and much more crude.
Smartphones can tell where you are through GPS or telephone signal towers triangulation.
Smartphones never turn off, most have a BIOS battery, meaning removing the battery does nothing for a while.
Most don't even have removable batteries.
All have bluetooth, which is an absurdly insecure technology.
Smartphones are almost always connected to the internet.

If you install LineageOS with NO google services, you will notice a two week battery life, or one week maybe.
This is because google services are surveillance.
Most apps will behave badly and some will stop working.
Banking apps refuse to work on rooted devices or those without google services installed.

> Smartwatches 
are the logical next step and are worse.
They know how many steps you've taken.
Your heartbeat.
What notifications you care about and which you won't even open.

They could predict illnesses.

> Smart-tvs
are less of a menace, as they do two things, provide mind-numbing entertainment, and are aware of what entertainment you watch.
Some have cameras though. Most have microphones on the controller. Those are always on.

In short, a meta-account is created for you out of all accounts and devices you use, including those without an account. 
Facebook calls them shadow-accounts, for people mentioned on their platform, but who haven't created an account yet.
And then, information is linked to it, collected, as if milking a cow. Because you are cattle to them.

Many services promise anonymity, by telling you your data is anonymized.
There exist paid services where data is cross-linked automatically and de-anonymized.
Your data may be linked to an anonymous number, but it is still identifying data.
After all, there is only one person in this planet with your exact behaviour patterns.

So, to stop most of the damage, stop using your smartphone for any nu-necessary tasks, stop engaging social media.