I have 2 LED strips with 6 LEDs on each. They use a 12V power supply.
My question, how can I use them like regular LEDs using Arduino UNO? Without any MOSFETS or transistors if possible.
And can I use PWM?
It's not safe for the MCU without transistors or so.
You can use some kind of LED driver (for example TLC5940 with on chip PWM) or ULN2003 as 8 channel low side switch (instead of 8 transistors, PWM is possible if you do it programmatically) or many others.
Where R_1 can be about 1K and transistor can be any NPN that could handle led current.