user51117: I'd like to connect a 12V access control device to the Arduino, but I lack the understanding.  
How can the signals from and to the device be done when they have different voltages?  
I am afraid that the 12V would brick the Arduino.

user6936: You could use an opto-coupler:

An opto-isolator (also called an optocoupler, photocoupler, or optical
isolator) is an electronic component that transfers electrical signals
between two isolated circuits by using light. Opto-isolators
prevent high voltages from affecting the system receiving the
signal. Commercially available opto-isolators withstand
input-to-output voltages up to 10 kV and voltage transients with
speeds up to 25 kV/μs.
A common type of opto-isolator consists of an LED and a
phototransistor in the same opaque package. Other types of
source-sensor combinations include LED-photodiode, LED-LASCR, and
lamp-photoresistor pairs. Usually opto-isolators transfer digital
(on-off) signals, but some techniques allow them to be used with
analog signals.

Here is an example schematic and packaging:

If you have a heavy load (although this does not seem to be your case) then they can also be used in conjunction with relays - here is an eight channel opto-coupler relay module board:





51117: <p>I'd like to connect a <a href="https://www.aliexpress.com/item/Security-RFID-Proximity-Entry-Door-Lock-Access-Control-System-Quality-5YOA/32283856982.html" rel="nofollow noreferrer">12V access control device</a> to the Arduino, but I lack the understanding.  </p>

<p>How can the signals from and to the device be done when they have different voltages?  </p>

<p>I am afraid that the 12V would brick the Arduino.</p>
 6936: <p>You could use an <a href="https://en.wikipedia.org/wiki/Opto-isolator" rel="nofollow noreferrer">opto-coupler</a>:</p>
<blockquote>
<p>An opto-isolator (also called an optocoupler, photocoupler, or optical
isolator) is an electronic component that transfers electrical signals
between two isolated circuits by using light. Opto-isolators
prevent high voltages from affecting the system receiving the
signal. Commercially available opto-isolators withstand
input-to-output voltages up to 10 kV and voltage transients with
speeds up to 25 kV/μs.</p>
<p>A common type of opto-isolator consists of an LED and a
phototransistor in the same opaque package. Other types of
source-sensor combinations include LED-photodiode, LED-LASCR, and
lamp-photoresistor pairs. Usually opto-isolators transfer digital
(on-off) signals, but some techniques allow them to be used with
analog signals.</p>
</blockquote>
<p>Here is an example schematic and packaging:</p>
<p><a href="https://i.stack.imgur.com/OJIy6.png" rel="nofollow noreferrer" title="Optocouple"><img src="https://i.stack.imgur.com/OJIy6.png" alt="Optocouple" title="Optocouple" /></a></p>
<p>If you have a heavy load (although this does not seem to be your case) then they can also be used in conjunction with relays - here is an eight channel opto-coupler relay module board:</p>
<blockquote>
<p><a href="https://i.stack.imgur.com/FBcjs.jpg" rel="nofollow noreferrer" title="12V 8 Channel Level Trigger Optocoupler Relay Module For Arduino"><img src="https://i.stack.imgur.com/FBcjs.jpg" alt="12V 8 Channel Level Trigger Optocoupler Relay Module For Arduino" title="12V 8 Channel Level Trigger Optocoupler Relay Module For Arduino" /></a></p>
<p><a href="https://i.stack.imgur.com/dauUX.png" rel="nofollow noreferrer" title="12V 8 Channel Level Trigger Optocoupler Relay Module For Arduino showing optocouples"><img src="https://i.stack.imgur.com/dauUX.png" alt="12V 8 Channel Level Trigger Optocoupler Relay Module For Arduino showing optocouples" title="12V 8 Channel Level Trigger Optocoupler Relay Module For Arduino showing optocouples" /></a></p>
</blockquote>
