Esp-01 Datasheet -

The ESP-01 is the perfect tool for one specific job: Turning "dumb" serial devices into smart devices.

Pro Tip: Keep a PDF of the ESP8266EX Technical Reference Manual on your desktop. You won't read all of it, but you will search for "Deep Sleep" and "GPIO" a thousand times. esp-01 datasheet

Note: GPIO 15 (TXD) must always be LOW during boot. This is why you can't just plug this into a USB-Serial adapter without a button circuit. The datasheet makes the ESP-01 look limited (only 2 usable I/O pins, needs level shifting, awkward boot sequence). And honestly, for a new project, you should probably buy an ESP32-C3 or an ESP8266 D1 Mini. The ESP-01 is the perfect tool for one

But trying to use an ESP-01 without consulting its datasheet is like trying to fly a drone in the dark. You might get it off the ground, but you’re going to crash. Note: GPIO 15 (TXD) must always be LOW during boot