Now that I found some old links... This began to be more public since Vizio published their 2021 Q3 earning reports and it was made public that their profits from advertising and users data was more than double than what they made selling devices [0]. Roku themselves have said that they are in the Ad business, not in the tech business, and they were getting around $40 per user a month (also in [0]).

All these ACR (automated content recognition) systems mix and match data from what they can see in your screen, with your IP address, with cookies you have on different devices... soy they can match different devices you own.

For example, AudienceX says about ACR [1]:

Second-Screen Experiences: Being able to synchronize content across connected devices is vital for successful advertising. ACR lets you do this by identifying the content that viewers engage with most on a primary screen (such as a smart TV) and delivering similar or the same content to secondary devices such as mobile phones or tablets.

[...] Automatic content recognition relies on two main types of technologies—fingerprinting and watermarking—to identify and analyze content. However, ACR technology can be broadly categorized into three main types. These include: Audio Fingerprinting [...] Video Fingerprinting [...] Digital Watermarking.

The Drum news site says [2]:

Automatic content recognition (ACR) is a technology built into smart TVs that allow the set to see or hear what is being played on the television. [...] ACR gives advertisers the keys to go beyond impressions, to understand who is watching and when. When paired with digital ad libraries, advertisers can find exactly where their ads ran – the hour, on what network or streaming service – and understand the exact corresponding impressions for the specific occurrence of their ads. [...] You can even go as deep as zip code, reading further between the lines about the impact of advertising at a local level.

Grapeseed Media, that works with agencies to provide technology and plans, says [3]:

For example, let’s say your client is Nike, and you’re running a Connected TV campaign for Nike shoes. When a person sees your Nike ad on their CTV, you want to deliver a specific set of display banners to their mobile devices so that the next time they open up a browser on their phone, they see these banners.

One of the ways to do this is with ACR. You would contact an ACR vendor and give them all of the details of your campaign, and the ACR vendor would use their software to create an audience segment of people who have been exposed to your Nike shoe ad on CTV [Connected TV]. Then, they would send you that audience segment so you can target it with banners. Think of this data like a file that can’t be opened — you can see the title, but you can’t dive in. You can only upload it and target whatever is inside.

All of this happens in real-time...

Mountain advertising software company say that they know all the devices in your household and will match your "Connected TV" ad impressions with your visits from others of your devices... [4]

So this is what they're doing with your info.

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0: https://www.theverge.com/2021/11/10/22773073/vizio-acr-advertising-inscape-data-privacy-q3-2021
1: https://audiencex.com/insights/automatic-content-recognition/
2: https://www.thedrum.com/opinion/2023/05/08/acr-data-the-key-measuring-more-just-impressions-ctv
3: https://grapeseedmedia.com/blog/programmatic-ad-strategy/
4: https://mountain.com/performance-tv/verified-visits/