If you do not define what type the code is in a fenced block, i.e. where you start the markdown with: ``` on its own on a line, the interpreter will try to guess what needs highlighting.
So if you use:
```
Your code block will look like this
exiftool "-alldates<datetimeoriginal" *.jpg -overwrite_original
exiftool "-filemodifydate<datetimeoriginal" *.jpg
but if you use
```text
it will look like this
exiftool "-alldates<datetimeoriginal" *.jpg -overwrite_original
exiftool "-filemodifydate<datetimeoriginal" *.jpg
although in Joplin the type for text is actually
```plaintext
If your block of code is css you use
```css
if it's html
```html
and so on...
This post and the one after it covers this in a bit more detail.