I tried to import CSV files to QGIS using the "Add delimited text layer" feature. For some reason, some of the numeric variables became QString.
How do I make them become integer so I can perform heatmap calculations?
You can enforce the data type behaviour you want with "Add delimited text layer" using a CSVT (my previous statement was it wasn't possible at all but due to Alexandre Neto feedback, I changed my answer)
You can follow this blog post to learn about CSVT files.
You can also add your CSV file with "Add Vector Layer" and use a CSVT file again.