PiP is not available when the main source is TV itself nor USB (the latter is a pity).
I used Samsung HU6900 PiP function to check the picture quality of my external STB Ferguson Ariva 253 Combo tuner. What I was very surprised, the picture quality (sharpness, colour saturation, contrast) of Ferguson Ariva 253 as a HDMI source on Samsung HU6900 4k UHD LED screen was superior to the picture quality of the original Samsung EU40HU6900 TV internal tuner. Might be Samsung HU6900 internal electronics does not work hard on PiP signal, or just Ferguson Ariva 253 (based on Ali CPU) has better picture processing

There is also another way to call PiP in HU6900 series: open Web browser (i.e. from on-screen remote), then in top right corner of the browser screen you find an icon of PiP. When you call PiP, then you get TV channel picture & sound when browsing.