If you are talking about python processor engine in 2.0 then basically it uses virtual environment library (venv ) which you should have it installed to you PYTHON_HOME path before enabling the python extension from the nifi.properties file ( see referenced guide).
When you deploy your python processor under {NIFI_PATH}/python/extensions and restart nifi , the nifi framework will process this file , move it to its own directory under {NIFI_PATH}/work/python then install python venv which will create a python isolated environment for that processor which will have its own python commands including the Pip command. Those python commands will be add to the bin (or script for windows) under the processor folder which will be used to download the dependency packages to that environment as well. Regarding defining the dependency packages, if you follow any existing python processor file template you will find them defined in the dependencies tag under ProcessorDetails class as follows:
class ProcessorDetails:
version = "2.0.0-SNAPSHOT"
description = """ Some Description"""
tags = ["excel", "json", "convert"]
dependencies = ['pandas','numpy','openpyxl',"xlrd"]
Once the venv is created , the python engine starts downloading those packages automatically and no interference is required. You can track the download progress in the app-nifi.log file. you can know that the dependency download is complete by either checking the processor status itself in the nifi canvas where all defined properties and relationships are listed, or you can look for the file ""dependency-download.complete" " under the processor folder where the packages are getting installed. If the file has the value of True then it means the dependencies have been downloaded successfully and the processor is ready to use.
Hope that helps.
