Should I use 'which' or 'that'?
Use that when – the information is essential to the sentence for it to make sense. It’s not separated by a comma. E.g. The company that invented the microchip we use invited us to a demonstration.
Use which when – the information is non-essential/extra, and separate with a comma. (I like to think of the commas as brackets, so if you removed that bit of information the sentence would still make sense.) E.g. Widgets Incorporated, which invented the microchip we use, has officially declared