The general rule is that you use ‘that’ in a part of a sentence that can’t be left out without affecting the meaning.
‘Birds that can’t fly are called flightless.’

If you remove ‘that can’t fly’, the sentence becomes ‘Birds are called flightless’, which is not true.

You use ‘which’ after a comma to add additional information to a sentence.
‘Emus, which lay green eggs, are flightless birds.’
(photo from Rex Boggs)
Photo: which or that? The general rule is that you use ‘that’ in a part of a sentence that can’t be left out without affecting the meaning. ‘Birds that can’t fly are called flightless.’ If you remove ‘that can’t fly’, the sentence becomes ‘Birds are called flightless’, which is not true. You use ‘which’ after a comma to add additional information to a sentence. ‘Emus, which lay green eggs, are flightless birds.’ (photo from Rex Boggs)