Google provides an overview of Transitioning from other computers to a Chromebook, but on some topics it's pretty general.
Step one, if one doesn't already have one, is to open a Google account.
Step two is
"Migrate your filesWhich appears to mean by download to copy files documents to the Google Drive folder on one's PC, and they'll be available on the Chromebook. [Update: here's how to Install Google Drive on your Mac/PC.]
"You probably have a bunch of files stored on other computers at home. Install Google Drive on your Mac or PC and download your essential documents, spreadsheets, and presentations. Then find your files in the Google Drive folder that is conveniently built into the Files app on your Chromebook."
Having a long-established website, I have a lot of web pages on my PC. I edit them offline and can proofread them offline before uploading them to my web host. Google says one can Host webpages with Drive by selecting the Share option (button) on a web page in Drive. What I'll need to investigate first is how to preview web pages in Drive using a web browser.
Over many years, I've accumulated a lot of digital photos on my PC. Derek Walter anticipated my question, Save Photos in Google+ or Google Drive? As of a couple years ago, at least, he suggested activating any available the Instant Update feature on a smartphone, which will put the photos in Google+. Presumably that would be the place to copy photos already on a PC, as well. After any editing of the photos, he then copies them from Google+ to Google Drive.
Contemplating the potential move of at least some of thousands of audio files lead me to Arpit Kumar's suggestion on how to Transfer Files To Chromebooks from Windows, Android and iOS by installing the Intel Easy Migration extension to the Chrome browser on both machines. My PC and Chromebook both have Intel processors, so I've done this.
Now it's just a matter of "pressing the button" and hoping I'm not overwhelmed by the transfered files.