Ubuntu used to send you a CD-ROM ready to use free of charge back in the day.

I've done this whole process hundreds of times and it's straightforward now, but the first times where really confusing, I totally understand you.

you basically want to put the installer file in a USB, but you can't simply copy and paste it because it needs to be "bootable", that's why you use a tool like Etcher or similar.

any link in the official Linux Mint page will point you to reputable sources so just pick the first one. the beauty of open source is that it's not as centralized as a big corporation's software, you have choices, confusing at first, I know, but it's worth it once you get past the learning stage