Its really hard to tell from a technical perspective, especially without having closely monitored all of your digital activity (and those that you have been in close contact with) in the days/weeks leading up receiving the ads. Some things that Meta could have done (in varying degrees of realism) include:
- read anything you downloaded from your Matrix client, like file attachments
- read your notifications if they contain any contents of the conversation
- read your clipboard if you copy/pasted anything into/out of a Matrix client
- actively participating in the room and associated your Matrix ID to your Meta account(s)
- scraped the contents of the room if it is public and unencrypted
- others in the Matrix room saved your Matrix ID in your contact information within their contacts
- Meta is recording your screen outside of Meta's apps
- a Meta library is used in another app/service on your device that is sharing information back to Meta
- read an attachment that you downloaded elsewhere then shared on Matrix
- Meta read screenshots you or others took of the conversation
- Meta has a back door in the Matrix server or client software used
- the administrators of your Matrix home server (or the administrors of any other home server in the room) are sharing non-encrypted information to Meta to offset hosting costs
- Meta is running a home server of a user in the room
- you or someone you are associated with clicked on a link shared in the Matrix room that contained a tracker or led to a site that contained a tracker
Its really hard to comprehensively and conclusively avoid all "spying" that Meta/Instagram could do to you. The best thing that you could do is something that many people aren't capable or willing to do - not install any Meta software, don't use any Meta services, block any Meta IP addresses and/or domain names, and advocate that those around you do the same.
Realistically, the best advice that youre going to get has already been said. Use the web browser instead of the app as much as possible, ideally in a different browser and/or user profile. If you must have the app installed, keep it in a separate profile and kill the app and/or profile whenever it is not in use. Review all of your security and privacy settings in all Meta apps. Review any apps/services you allowed Meta to connect to/from (and the security/privacy settings of those apps). Reduce the amount of information that you enter/share on Meta platforms. Review the other users that you are connected with on Meta's platforms.
I did not watch the mentioned video so I am not sure if what I am about to mention is discussed there or not. Also, sorry for the really long reply!
I am not aware of any available truly privacy respecting, modern cars. However, assuming theat you obtain one or you can do things like physically disconnect/remove all wireless connectivity from the car to make it as private/secure as possible, there still is little you can do to be truly anonymous.
Your car likely has a VIN and license plate as well as a vehicle registration. Assuming you legally obtained the vehicle and did not take any preventative measures prior to purchasing the car, those pieces of information will be tied back to you and your home address (or at least someone closely connected to you). You would need to initially obtain the vehicle via a compsy/LLC/partnership/etc. as the owner/renter/leasee of the vehicle and an address not associated to you. Additionally, you would need to find some means of avoiding or limiting the additional information connected to you that is likely required to obtain the vehicle like car insurance and your drivers license.
Additionally, any work that certain mechanics perform may be shared (either directly or indirectly) with data brokers - even just routine maintenance like an oil change or alignment. Hopefully you didn't use your credit card, loyalty rewards program, etc. when you had any work done!
There is also CCTV, security cameras, and other video recorders that are nearly impossible to avoid. Given enough time/resources and maybe a little bit of information, your car could be tracked from its origin to destination locations. This location history can be used to identify you as the owner (or at least driver/passenger) of the car. Unless your car never leaves your garage, you can almost guarantee that your car is on some Ring camera, street camera, etc.
Furthermore, anything special or different about your car (custom decal, unusual window tinting, funny bumper sticker, uncommon color for the car, uncommon trim/package for the car, dented bumper, fancy rims, replaced tires, specific location of toll reader placement on the windshield, something hanging from your rear mirror, etc.) all help identify your car. The make/model and year of your car can also be used to identify your car if its not a common car in the area. These identifiers can be used to help track your car via the video feeds mentioned above.
Then there are license plate readers which are only slightly easier to avoid than the video recordings. Permanent, stationary license plate readers can be found on various public roads and parking lots. There are also people who drive around with license plate readers as part of their job for insurance/repossession purposes. You may be able to use some sort of cover over your license plate(s) to hinder the ability of license plate readers to capture your plate number, but that could be used to help identify your car in video feeds/recordings.