And can anyone specify what are ways this ripples can be reduced? You could use an LDO or TVS diode to reduce the spikes. But better to look first at your 3.3v regulator, why is it passing such spikes through. Is the feedback loop node receiving noise? Is the output capacitance too low? Need to see...

If the supply.voltage is 3.3v it (most likely) won't output any higher than that. Separate parts of the system, figure out what isn't working. Can you use a multimeter or oscilloscope to read the analog voltage of the soil moisture sensor? Can you put an intermediate 0-3.3v on the ADC pin and read a...

Maybe you need something like

What is your 5v phone charger rated for? 500mA, 1A, 2A?

Are capacitors close to the esp32 power supply pins? 100nF and 1uF

See https://github.com/espressif/arduino-esp32/issues/1804

1.5 / 3.9 * 4095 = 1575, it's about what you'd expect

Esp32 pico with it's integrated crystal and flash will be smaller than standalone chip

The recommended min power supply is 500mA 3.3V. Is your power supply smaller than that, causing the brown outs?
Little larger power supply and/or more output capacitance can help avoid the voltage dip brown out.

Use a DC Boost converter to provide the proper voltage. Do you have size / cost / thermal constraints that prevent doing that?

You can try blynk to handle the phone to esp32 communication
https://blynk.io/en/getting-started

5V on a dev board is straight from USB, that can supply power. But esp32 is a 3.3v device, don't put 5v on any gpio pin. If you can, supply the pot with 3.3v. Some of the gpio have multiple purposes, like strapping pins, their value is read at boot up to set some settings. So if you have other stuff...