TVs that had PIP back in the day (when it was common) had two antenna/cable inputs and the TV had 2 separate tuners inside. That allowed you to tune into 2 different channels at the same time. Nowadays, unless you're using an OTA antenna and want PIP on two different OTA stations, you'd be looking for something a little different. If you have cable or satellite, you almost definitely have a cable box or satellite receiver hooked up to the TV. (I'm not aware of any cable company that doesn't require boxes). The PIP function would really have to come from the box, not the TV. The box does all the work tuning into the stations. It then sends out "dumb" video and audio signals that the TV receives and plays back. It's like playing a DVD or Blu-Ray: the Blu-Ray player does all the work reading the disc; the TV just plays whatever the Blu-Ray player tells it to. Same thing with a cable/satellite box. You'd have to see if your cable/satellite provider offers PIP.
Now, as for having PIP via two different HDMI inputs, or PIP on one input and an app, that may or may not be available on some TVs; I haven't seen or heard of it before though.