how to check space on startup disk

One popular app to use is OmniDiskSweeper.

https://www.omnigroup.com/more/

Click the Apple menu top left.

Select About this Mac

Select 'Storage'

Select 'Manage' top right of window

(The buttons in the right hand section - do not click them unless you are aware of what they do)

In the left hand section you can click the different types of file on your system, and you can locate them and delete them as you need - when you hover over them a 'x' (delete) and spyglass (locate) appears.