In addition to what the previous answers mentioned, virtually anything on your hard drive was, at least at some point, also in your RAM. Writes to (and reads from) disk virtually always use what is called Direct Memory Access (DMA), where the CPU commands the DMA controller to transfer a block of memory between some device and RAM. Virtually all disk accesses on modern computers happen this way, where the CPU will issue a command to transfer blocks of data from the disk to RAM or from RAM to the disk.
With DMA transfers, the CPU just gets an interrupt when the process is complete. The reason this is done is so that the CPU is not tied up during the transfer and can be busy doing other things (or else can be sleeping and not wasting power.) Virtually all I/O and storage devices (including even the RAM, but especially the disks) are much, much slower than the CPU. While we're still talking about relatively small fractions of a second, the time it takes to transfer a block of data from RAM to a hard drive may as well be an eternity from the CPU's perspective. So, allowing the CPU to just command the process to start and get an interrupt when it's done saves tons of time that can be used for other useful purposes on the CPU.
When you loaded a webpage in your browser, the Ethernet controller would have placed the data coming from the website into your RAM, probably still in an encrypted format. From there, it would be decrypted into another RAM buffer. The browser would generally use the buffer already in RAM to display the page, but then also copies it out to disk using DMA so that it can load it from there again if it needs that resource again instead of having to download it from the Internet again.