Even though not every detail is relevant for this question, I will list my setup nonetheless:
To compile my code and flash the binary to the chip, I issue the following command in my cmd terminal:
> mbed compile -t GCC_ARM -m NUCLEO_F746ZG --flash
I get the following output:
...
+------------------+-------+-------+-------+
| Module | .text | .data | .bss |
+------------------+-------+-------+-------+
| [fill] | 130 | 4 | 10 |
| [lib]\c.a | 24965 | 2472 | 89 |
| [lib]\gcc.a | 3120 | 0 | 0 |
| [lib]\misc | 252 | 16 | 28 |
| mbed-os\drivers | 658 | 4 | 100 |
| mbed-os\features | 74 | 0 | 12556 |
| mbed-os\hal | 2634 | 4 | 66 |
| mbed-os\platform | 2977 | 4 | 270 |
| mbed-os\rtos | 15887 | 168 | 5989 |
| mbed-os\targets | 16013 | 4 | 1052 |
| source\main.o | 244 | 4 | 84 |
| Subtotals | 66954 | 2680 | 20244 |
+------------------+-------+-------+-------+
Total Static RAM memory (data + bss): 22924 bytes
Total Flash memory (text + data): 69634 bytes
Image: .\BUILD\NUCLEO_F746ZG\GCC_ARM\nucleo_f746zg_demo.bin
[mbed] Detected "NUCLEO_F746ZG" connected to "E:" and using com port "COM10"
1 file(s) copied.
I'm particularly interested in the last lines, where the actual flashing of the chip takes place:
Image: .\BUILD\NUCLEO_F746ZG\GCC_ARM\nucleo_f746zg_demo.bin
[mbed] Detected "NUCLEO_F746ZG" connected to "E:" and using com port "COM10"
1 file(s) copied.
I know from previous experience (before mbed CLI existed) that there is a lot going on to flash a binary to a chip. For example, I had to startup openocd, pass it a configuration file of the programmer (eg. stlink-v2-1.cfg) and a configuration file of the target board (eg. nucleo_f746zg.cfg). At last, I had to hand over the binary to openocd via a Telnet-session or a GDB-session. Everything is described extensively here: How to use the GDB (Gnu Debugger) and OpenOCD for microcontroller debugging - from the terminal?
Looking at mbed CLI flashing the chip, I get confused. What is happening on the background? Is mbed CLI secretly using openocd to connect to the chip? Or perhaps pyOCD? Or some other way?
mbed-cli is open source, you can find the repository here. If you search for "def compile_" you'll find the specific code for what is happening when you run mbed compile.
mbed-cli uses mbed-ls to detect your board and htrun to flash it. htrun has a variety of plugins for copying to different boards, including pyocd but in the most basic case it just copies to the disk detected with mbed-ls.