The TI84+ calculator is using the Zilog Z80 processor.
This processor is using the BCD-Arithmetic.

Is the TI84+ also using the BCD-Arithemtic ?
Or am I completly wrong?

It's a little misleading to say the processor is using BCD arithmetic- the processor does normal, byte arithmetic and TI-OS transforms the result using the daa instruction. The important distinction here is that you cannot treat all z80 assembly code as though it acts on BCD integers.

BCD arithmetic is widely used in calculators because it behaves similar to how a human would. This is important for, say, financial or scientific calculations where large numbers need to be considered with high accuracy. So yes, the TI-84+ uses BCD arithmetic for numbers. Specifically, it uses a float with an integer exponent and a 14-digit two-digits-per-byte binary-coded-decimal mantissa. You can read more about this format in the link guide, and I've implemented standard operations on these floats here.