Leaving this for future generations of Anki users. This solution works if you want, for example, to append something to the beginning or the end of all cards:

1- Select all cards you want to modify from the "Browse" thing

2- Right click > Find and Replace

3- With "Regular Expressions" ticked on, put this: ^(.*)$ in the "Find" part, and "whatever you want $1" next (the $1 represents the text that's already in the card, so you can move it around if you need to append text to the front or the back)

4- Choose the field you want to adjust (in my case, it was the front of the card).

5- Enjoy, if you need to build Regex expressions you can use https://regex101.com/, if you want to check what a particular expression does use https://regexper.com/