Fair questions, they confused me as well when I started.
The input deliberately draws a few mA when it’s on. An input that only « measured voltage » with a very high impedance would see 15 V picked up on a long cable next to a motor lead and call it ON. By needing 3 mA, a weak induced voltage can’t hold it up. That’s what the type 1/3 classes in IEC 61131-2 define: voltage and current ranges for ON and OFF.
It also matters for 2-wire sensors. A 2-wire prox still leaks a little current when it’s off. If that leakage is above what the input accepts as OFF, the input never turns off. Type 3 inputs are designed to tolerate that leakage, type 1 inputs may not be.
Your wiring: « sinking » input means current flows into the input and out to 0 V, so it needs a device that sources +24 V into it. That’s a PNP sensor or a dry contact between +24 V and the input. An NPN sensor pulls the input towards 0 V, so on a sinking input it does nothing. Options: buy PNP versions of the prox sensors (easiest), or use an interposing relay.
The door contact and float switch are dry contacts: wire them from +24 V to the input. 3 mA through a reed contact is fine.
On the Hermetry side send the states as 0/1 numbers so they can be charted and alarmed on.