How can STM32F446RE connect external SDRAM

While the datasheet indeed says that, it's more that you simply don't have enough FMC-connected pins on the 64-pin package to make any reasonable connection.

You still can connect a SRAM and simply manually 'bit-bang' the read and write signals. Performance of course will be slow. SDRAM might be possible with a serious amount of hacking, but not practical at all.

There are also serially-connectible (SPI) SRAMs, e.g. from Microchip and ONSemi; again there is surely a need for some programming and some performance loss.

JW