Is somebody casting YouTube onto one of your devices?
I haven't used Google Home to control this kind of thing (But, I think if you
1) click on the icon in the top right corner,
2) Assistant settings
you'll see devices linked to your account - and you can remove them.
Also, I think if you go one level up from there (under "YOU" (Your information and personal preferences), you can go to "your people" and see if anyone you didn't expect is in your group
Are you using Wi-Fi? And do have an app that controls your wi-fi?
I was using Netgear/Nighthawk until recently, and I could control every device that had access to my wifi from there (I could block any MAC address I didn't recognize)
And now I am using Eero for wi-fi, and that app also allows me to block any device.
The hard part is getting a map of all the devices I have. I created a google spreadsheet that details the MAC address of every device that I have - but this isn't easy (I have 4 teenagers, and with all the computers, phones, televisions, dvd players, smart speakers, smart switches, door bells, wi-fi satellites, and even my appliances, - all have their own unique MAC address. I have well over 75 devices now. So, it's not always easy to tell if a new device is legitimate or not.
Fortunately, my Eero app allows me to name them. And my google spreadsheet allows me to track them all easily).
Sorry for the late response, but I wanted to drop in here to help.
I understand that these issues can be confusing but I definitely wanted to stop by and see how I could help.
Just wanted to double check to see if you were still needing help on this?
I look forward to hearing from you.