The monitor needs to have decent display quality but does not need to have anything fancy in terms of graphics since it is not bieng used for high quality videos or gaming. A suitable monitor should cost around $120 (as of 2019).
It should be as small as possible within reason (15 to 17 inches) so that it doesn't look like a wall-mounted television screen. It does need to have and HDMI connector which is what is on the Raspberry Pi board
It gets physcially removed from its surrounding frame and stand so that it can be fitted directly behind the mat and secured to the surrounding frame.
Almost all monitors now want 120 volts power.
When the monitor is removed from its frame there will generally be control buttons. These need to remain connected to the monitor and be fastened somehow to the inside of the frame. Normally they will not be used.