I just recently got this app and it’s perfect for shooting candids. It’s called SP camera it is $9.99 but it’s definitely worth it. You can make your phone screen black and press the volume button to start recording you can also put in any website use the website functional and also choose when to start the video on command while still being on the website. It stores the videos/pictures into the app not directly into your iPhone photos and can download them to your phone whenever you want. Trust me it’s definitely worth it
If you don’t want to download an app, save some money, and have an iPhone, you can do this.
Settings > Accessibility > Accessibility shortcut > voiceover
Then anytime you wanna record without your screen on, go start a video, triple click the power button, then triple tap with 3 fingers and the display shuts off but keeps recording. Then to stop you do the inverse, triple tap with 3 fingers, triple click the power button, stop video.
*make sure to have your volume down because the voiceover is true to its name and has audio
You can also switch the front camera and back camera with the invisible buttons on the website.
There are also existing apps, free.
See this thread, I know I have another, not sure where