The underlying cause(s) of the issues you describe can't be addressed briefly, but some simple techniques.
Business meetings:
In general, just having something to say doesn't mean you should say it. It's a group activity with a specific purpose (or, if you're working in an ineffective organization, you pretend that's the case) - if you're not
definitely contributing to the objective, say nothing.
So you don't track the sentences or sub-topics. You track what's happening against the meeting objective. As a junior, if it's moving forward, or they're just wasting time "flexing", say nothing.
- If you expect to provide necessary information, or ask a specific necessary question, write down the things you need to address (limit each point to one to four words - it's just a reminder & checklist).
- Remember it's a group activity. Unless you own the project/activity, you speak when you add business value. That's not the same as not agreeing with something, or adding a minor detail, or another angle on a topic.
Online business meetings:
- Turn off your mike. Don't turn it on unless you're sure you should be saying something (see above for some guidelines). This provides an automatic break in the "rush to say what just came into your head" process, where you can review the context.
Informally, with people (lunch breaks etc):
I'd need a lot more contextual information, but something to consider: the process isn't all that different from business meetings, except the tangible objectives vary (the topic of conversation will jump around).
One thing that might help (though it doesn't actually address your specific question(s) - try to track the two key things that don't require you to "read" the group:
- What is the current speaker trying to achieve?
- How are they using the conversation to do that?
The answers to both may be trivial BTW - e.g. it might just be someone who likes to talk and to dominate conversations describing something trivial. But in that case at least you know that they don't want to be interrupted
Note that this doesn't directly help you know what to say. when, or how. It's equivalent to your "meeting prep" notes, or turning your microphone off, but with a different upside: it will help identify places in the flow where your input would be welcome.