How much storage do you actually need on a computer or phone?

For a phone: 256GB is the comfortable minimum unless you record lots of video. For a computer: 512GB minimum, 1TB is better if you can afford it. Storage gets full faster than you think, so buy more than you think you need. You can't upgrade it later.

Storage requirements depend on what you keep on your device. The operating system takes up space, apps take space, and photos and videos take lots of space. Cloud storage (Google Photos, iCloud, Spotify streaming) reduces your local storage needs. But storage cannot be upgraded on most modern phones and laptops, so choosing the wrong size at purchase is permanent and expensive to fix.