First up, some files are hidden for a reason. Most of the time it's done to keep people out of critical, operational-sensitive files & locations that could brick a PC if tampered with.
Second, the method to unhide hidden folders will vary by OS. Windows, you'll have an option with the file explorer to "Show Hidden Files". In Windows 10, you can go to View & then check the "hidden files" button. Hidden files & folders will appear semi-transparent when this option is selected.
Android OS can hide directories from MOST applications by starting the folder name with a period (like ".private"). File explorers may ignore this convention.
Linux & Mac OS will require you to toggle a setting through the OS terminal (the command line).
Please be aware that this is only as the OS level... It's possible for people to hide files through other means, this is typically done one of two ways...
Method A) Merging an archive into an image file -- This is a bit technical & isn't perfectly foolproof (as somebody who looks at file sizes can discover a discrepancy between the image & it's file size, especially if something that should only take 1 MB is coming up 100 MB or larger), but the files within the archive are hidden behind the image. Those who view the file will only see the image, but will only find the hidden files when ran through a file archival tool.
Method B) File Encryption with a security container -- This is generally easier to create & possible detect, but much harder to uncover. Various file encryption tools, like VeraCrypt (the practical successor to TrueCrypt) create a encrypted container that appears to be like a typical file, but will typically use a file extension that's not typically used so it cannot be viewed by other files. However, to view the files within these containers, you will need the security passphrase (& in some cases, a keyfile) to decrypt them. There can be hidden containers, which can utilize a different passphrase (& keyfile) to access the files within the same container.
Considering these are user-generated hidden files, you (or another person who accesses your computer) would have created these encrypted containers... so these aren't hidden by the computer (unless you choose to make it hidden by the OS).
As mentioned above, it's highly likely that there are some hidden files on your system by default... but I wouldn't necessarily go poking around unless you know what you're doing.