I have downloaded OS MasterMap stylesheets from OS/Github.
How do I apply them in QGIS, all I get is style code rules for each symbology? see below picture example from Topo area file.
Your screenshot looks like you're most of the way there, the Rule column is looking at the
style_code field and saying every feature with the attribute of 1 will be displayed as a Building.
Of course, this assumes you have data in the correct format and has the relevant fields in the attribute table - which it should be if it has been provided by the OS, but is not clear from your screenshot.
Take a look at this doc from the OS for more info:
It goes through the steps required to apply the .QML files (stylesheets) from GitHub to your data, and will hopefully clarify if you're still having issues.