I have an iPhone 7 and I save all my work projects on it. On my iPhone I don't have any pictures or games but the memory is getting full and I'd like to free it. On my iPhone there are some preinstalled iOS apps that I don't use and they fill the memory. I would like to delete them but I don't know how to do that. How can I remove preinstalled iOS apps to get a faster system? Can I remove these apps or only the folders?
You can delete preinstalled iOS apps by holding down on the app icon until they wiggle, then choosing the × button. However, note that this will not free any space on your device as the apps are part of iOS and their files are not removed from your device. Removing any apps, regardless of whether they are part of the OS or not will not increase the speed of your device.
You can determine what is taking space on your device in Settings → General → Storage & iCloud Usage → Storage: Manage Storage.