Code coverage is stored in two Lightning Platform Tooling API objects: 

1. ApexCodeCoverageAggregate:

ApexCodeCoverageAggregate stores the sum of covered lines for a class after checking all test methods that test it. 

2. ApexCodeCoverage:

ApexCodeCoverage stores the lines that are covered and uncovered by each individual test method.

Reference Article – https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_code_coverage_intro.htm

1. Query:

SELECT ApexClassOrTrigger.Name, NumLinesCovered, NumLinesUncovered 
FROM ApexCodeCoverageAggregate 
ORDER BY ApexClassOrTrigger.Name ASC

Output:

2. Query:

SELECT ApexClassOrTrigger.Name, NumLinesCovered, NumLinesUncovered 
FROM ApexCodeCoverage

Test class in Developer console always shows None issue

Apex Test Class in Developer console always shows None issue