A three-phase voltage inverter is a circuit composed of six transistors pairwise (or an integer multiple of six if they are connected in parallel or, rarely, mostly in older designs, in series) where each pair uses pulse-width modulation to generate a particular average phase voltage. You will find more info at Power inverter - Wikipedia.
It’s the same thing. Conventionally, the inverter supply voltage is denoted Vbat.
It means that all six transistors are permanently closed. When turned off, the inverter may still conduct through the anti-parallel diodes.
Usually, it’s a set of TVS (transient voltage suppression) diodes, possibly with a braking resistor. You can find an example on the schematic here (the three diodes next to the power input): How I Learned to Stop Worrying and Love ESC Development - #4 by alexander.sysoev