Arduino board has a microcontroller called Atmega328. I wonder if it is CMOS or TTL. I dont know how to understand it from the datasheet. Second question is on the Arduino board there are other chips. Are they all TTL or CMOS? How can I know?
All the key facts about a chip should be contained in its datasheet. This should always be your first port of call when researching things.
Search for "CMOS". First hit is page 5:
"The ATmega48PA/88PA/168PA/328P is a low-power CMOS 8-bit microcontroller based on the AVR enhanced RISC architecture".
Bonus answer: a quick look at the Arduino UNO schematic shows three different voltage regulators, and an LM358D used as a comparator, all of which are considered "analog" parts and neither CMOS nor TTL.
The Atmega328 processor is CMOS. The other stuff is whatever the board designers decided to use, but most likely CMOS. However, instead of asking for heresay, go look at the schematic and BOM and see directly.