I realize that a very good answer has been given, but I'd just like to point out (because I'm learning about Power Automate and object-oriented programming too, and this was a revelation to me) that you have posted the text for an Array of Objects. This language should be helpful when searching for similar answers.
In JSON, everything is an Object initially, so I'm guessing you didn't copy over the initial curly braces. So what you really have is an Object containing an Array of Objects. You access an Object's properties (and sub properties) in Power Automate by something like this:
outputs('Name_of_Connector')?['Property_Name'][arrayintegerifnecessary]?['Subproperty_Name'] etc. The "?" shows the downward connection of objects and properties, which can be linked together, and when you put "[ ]" at the end of a property you're accessing that index of an array.
{ <---- this curly brace starts any JSON
"projects": [ <---- this open bracket indicates that "projects" is an array
{ <---- this open curly brace indicates the following text is an object
"project_id": 9, <---- this is the first Object property, which is an integer
"project_name": "Language processing", <---- this is the second Object property, which is a string
"project_description":
"{\"blocks\":[{\"key\":\"dopmg\",\"text\":\"XYZ\",\"type\":\"sharp\",\"depth\":5,\"entityRanges\":[],\"data\":
{}}],\"entityMap\":{}}", <---- this is also a string, though it looks to be an array of objects that has been turned
into a string)
}
]
} <---- this curly brace is at the close of any JSON
So, if a connector named "Compose_2" contained your JSON, you could access "project_id" with this text:
outputs('Compose_2')?['projects'][0]?['project_id']
And using the json() function to convert your Object's index-0-Array's Object's third string-Property, as @grantjenkins demonstrated, you could access XYZ like this:
json(outputs('Compose_2')?['projects'][0]?['project_description'])?['blocks'][0]?['text']
Getting the gist of all that helps immensely when plinking around in Power Automate.