I have a preference for one form or the other, depending on the context.
For case 1 about working at McDonalds:
1a) I worked at McDonalds as a teenager.
I would say this when the emphasis is on the fact that it was McDonald's and not somewhere else. For example:
Don't worry, I know all about making french fries. I worked at McDonalds as a teenager, you know.
In this case, the focus is on the fact that you worked there, and that fact matters for the point you're trying to make.
On the other hand, the "used to" construction makes the point more neutrally, or weakly. As @ryang says, the "used to" clearly implies that you're referring to a past condition which is no longer the case. Because of this, you can use it to introduce distance, or mildly downplay something. An example of this:
Yeah, I did use to work at McDonalds, but only as a teenager.
(Note here that the past tense is indicated on the verb "did", rather than "use". "I did used to work there" would sound alarmingly wrong.)
As a counterexample, suppose your friend is challenging you.
Friend:
You've never had to work a crummy job in your life!
You:
I worked at McDonalds as a teenager.
The plain form rather than the used-to form is a much more direct way to refute the challenge.
Regarding your comment about working at Google:
If you want to simply and neutrally tell someone you worked at Google, you would just say: "I used to work at Google".
If on the other hand you're trying to assert that you're a good engineer or something like that, you would use the plain form "I worked at Google", much like the making fries example. This form sounds a lot pushier to me.
You can however soften this plain form by adding a time statement to it. These all sound totally neutral to me:
- "I worked at Google for a while"
- "I worked at Google from 2017 to 2021"
- "I worked at Google a long time ago"
The reason these things sound softer is that the time statement indicates the exact same thing that the used-to form does - it puts the condition in the past and underlines that it's not the case anymore.
(Finally, just in the spirit of helping out, there are a couple small mistakes in your question:
"You're telling about it to a friend" is understandable but not something a native speaker would say. I might say:
- "You're talking about it to a friend"
- "You're telling a friend about it"
Likewise with "You no longer doing it". I might say:
- "You're no longer doing it"
- "You no longer do it"