For some strange reason, Apple has so far only released the videos of their Tech Talks in the WWDC app (and on the Apple TV - which I don't have). What if you want to watch them from the comfort of your sofa? Your hands are occupied already right🍕🍺?
Presumably they'll be on the website sooner rather than later, but in the meantime you can easily stream the content to your Mac using the excellent third-party AirServer (conveniently free for seven days). Once it's running, play the WWDC app video on your phone and, via the magic of AirPlay, they'll show on your laptop screen. If you can plug that into the TV, all the better.
Pro-tip: if you use the Share Sheet in AirServer, you even get the direct URL of the talk and can use it directly, dispensing with AirPlay completely.