How do I learn to properly break one liners into their respective single steps?

In addition to solution #1 by OriginalGriff, i would like to explain your problem in a bit different way. First of all, you have to understand what such of long line of code is doing to be able to split it into smaller pieces.
This:

return String.Join(".", (input.Split('.').Select(x => Convert.ToString(Int32.Parse(x), 2).PadLeft(8, '0'))).ToArray());

actually means: join strings from a text splited by dots and converted into zero-one form.

There's one reduntant call: ToArray(), because string.Join() method accepts IEnumerable(Of T) as an input. See: String.Join Method (System) | Microsoft Docs[^]

How to write it line by line? Simple. See:

string input = "192.168.0.1";
//split input into parts
var result = input.Split('.');
//returns an array os strings:
//192 
//168 
//0 
//1 

//convert each part
var result1 = result.Select(x =>Convert.ToString(Int32.Parse(x), 2).PadLeft(8, '0'));
//returns an IEnumerable<string>
//11000000 
//10101000 
//00000000 
//00000001 

//join strings
var result2= string.Join(".", result1);
//returns a string
//11000000.10101000.00000000.00000001

Finall note: C# is case sensitive. So, String is not the same as string. An explanation you'll find here: Strings - C# Programming Guide | Microsoft Docs[^]

solution 1 from OriginalGriff explains things very nicely
and solution 2 from Maciej Los also very good
and not to mention PIEBALDconsult comment "Any way you like" - nice :)

But possibly to help figure out how to get to solution 1 and 2.

One of the tricky parts I suppose in learning the language at hand is figuring out which of the words/bits belong to the language and which ones do not.

So in deciphering a one-liner, you will be required to do a lot of googling or possibly have a language reference at hand.

To simplify the one-liner, you could try adding carriage returns and indenting to help visualize better.

In the case of C#, separate by parenthesis, the comma and the dot.

//expanded one-liner - something like this
return String.Join(
    ".", 
    (
        input.Split('.')
            .Select( x => 
                Convert.ToString(
                    Int32.Parse(x),
                    2
                )
                .PadLeft(8,'0')
            )
    )
    .ToArray()
);

Do searches with the program language of the one-liner - in this case C#
Let see what google search results we can get from above (if you know what they do already - no need to search):
return c# return statement - C# Reference | Microsoft Docs[^]
String c# String Class (System) | Microsoft Docs[^]
String.Join c# OR Join c# String.Join Method (System) | Microsoft Docs[^]
input.Split c# OR Split c# How to: Parse Strings Using String.Split (C# Guide) | Microsoft Docs[^]
.Select( x => c# c# - Possible purpose of LINQ select( x => x ) - Stack Overflow[^]
Convert.ToString c# Convert.ToString Method (System) | Microsoft Docs[^]
Int32.Parse c# Int32.Parse Method (System) | Microsoft Docs[^]
PadLeft c# String.PadLeft Method (System) | Microsoft Docs[^]
ToArray() c# List<T>.ToArray Method (System.Collections.Generic) | Microsoft Docs[^]

Add comments to the expanded one-liner - getting the information from the google searches.

return String.Join(//return a string of Concatenates the elements of a specified array or the members of a collection, using the specified separator between each element or member 
    ".", //using this separator
    (
        input.Split('.')//creates an array of substrings by splitting the input string based on one or more delimiters - using a separator as '.'
            .Select( x => //this is a tricky one - use linq select to parse the values somehow
                Convert.ToString(//Converts the specified value to its equivalent string representation
                    Int32.Parse(x),//Converts the string representation of a number to its 32-bit signed integer equivalent
                    2//Converts the value of a 32-bit signed integer to its equivalent string representation in a specified base
                )
                .PadLeft(8,'0')//returns a new string that right-aligns the characters in this instance by padding them on the left with a specified Unicode character, for a specified total length
            )
    )
    .ToArray()//An array containing copies of the elements of the List<T>
);

From this point - hopefully you will be able to figure out what parts can be broken down.
And expand to what you could get in Solution 1 and 2.


You may have noticed I didn't separate out all the words:
- String.Join
- input.Split
- Convert.ToString
- Int32.Parse
see what you get from searching them split up or not