My partner has a old 2003 Ford Focus and I want to change the radio from stock to something that has bluetooth.
Here's my understanding on aftermarket radio:

• Every car is different, depends on the dashboard and the radio harness thing
• Finding out what what radio to buy/how to fit it is usually done by finding old YouTube videos from someone with 100 followers from 2008
• Sometimes it can be really easy and sometimes it can be really annoying to fit

Is my understanding correct?
I've tried this before with another car I had (a 2009 Mitsubishu Lancer) and it was such a nightmare I gave up. So I want to double check with any car nerds that I'm not missing anything obvious or there's no source of truth or something to go checkout/learn.

Any help would be appreciated, otherwise I'm going to go and find some 180p videos from 2008 to watch.

Edit @smithsonian, @Weldawadyathink, @scottc
Thabks for your replies, the site looks great for what I'm looking for. I'm not US but they do ship internationally. Plus it will at least help me work out what parts I might need!
Glad something like this exists, thank god.