What programming methods can this chip support? Is there serial programming or SWD programming?
The Arduino IDE (a free download) supports programming in C++. The IDE pre-processes your code (which they call a "sketch") slightly but it is still C++.
You can drop into assembler if you absolutely must, but the C++ compiler does a good job of ...
Are the ATMEGA and STM32 programmers not universal?
They are not the same. As far as I know their programming arrangements are completely distinct.
What programming methods can this chip support?
Traditional AVR In-Circuit-Serial-Programming (ICSP) using a 2x3 header pin connector and one of a variety of programmers such as the STK500 which has low-cost ...