At the heart of the ESP-01 module is the ESP8266 chip with at least two boot modes — the normal mode and the programming/flash mode. The latter refers to uploading any software (or firmware) to the ...

However note that the ESP-01 board will have a red LED turned on indicating the presence of power while in the ESP-01S board, you will only see a blue LED flash for a while when the power is applied.

For over-the-air programming, or to give you some more space ... This is especially easy if you’ve got an ESP-01, ESP-03, or ESP-11 modules where the flash chip is exposed.