Expected to start teams meetings?

It’s not that deep.

Turn up to the meeting on time. If you’re first, start the meeting. If you’re a couple of minutes late, apologise.

If it’s the culture in that place that the most junior person starts the meeting then just go with it. You are junior to your boss, and that’s the way they do things where you now work.

But he wasn't "just sat there" was he? Wasn't he in another meeting?

In my organisation you join at the hour specified unless you are in another meeting that is overrunning. If you are first to join, you sit in the meeting with your camera off doing other tasks (like your training materials) while you wait for the back to back overrunning person to join.

It's not about who's more junior, it's about who is coming from another meeting (possibly not in control of their schedule, can be a few mins late) and who is breaking off their work to join the meeting (in control of their schedule, should be punctual).

OP just take a deep breath and do as you're asked?

Someone has to start a meeting and he's decided it's you.

It's really no big deal.

Your manager can do what he likes, he's THE MANAGER. You've been directed to join the meeting at the start time, so that's what you do. To be honest it's what most people would logically do anyway but as he's had to spell it out to you I'd just do it, unless you have a genuine reason not to e.g. previous meeting overrunning.

As others have said it doesn't matter what the other attendees do if you're free to log on at the start time then you log on.

Join at the start time, shows you are punctual, and gives your manager or others an excuse to leave the overrunning meeting.

And if you have several people and one is late, start without them. None of this 'give it a couple of minutes'.

I host a meeting, it starts on time, and almost no-one is late because they know it will start on time.

Oh dear. Good luck in your new job if this is your attitude.

I don’t see you lasting long in this job. You’re not exactly great at communicating if this thread is anything to go by, you’re very aggressive and rude. And you’re arguing back against your senior instead of just doing what you’ve been told.

Why is this confusing?
He was already in a meeting.
He couldn't start your meeting if he was already in one.
Whereas you could start the meeting because you werent in one.
If you had a valid reason for joining late, he knew he could carry on with the first meeting until you were ready. However if you joined your meeting at the right start time he would get a notification that you were waiting and could wrap up the first meeting if it was overrunning.
Otherwise he thinks "oh x hasn't started the meeting she can't be ready yet so I'll carry on." (Or go for a quick loo break)= your meeting starting late for no reason, which had a knock on effect on all his other meetings that day.

Simply put, he is the manager, his time is considered more important than junior staff (particularly new starters without any actual work on yet), it makes more sense for you to hang around waiting for a few minutes for him than vice versa.

Pretty much everyone on this thread has told you it's common practice, at the end of the day it doesn't matter whether you think its stupid or unfair - that's what your manager wants, you presumably don't want to piss them off for something so minor, so just do it!

Oh right. Yes he can / should just as easily start the meeting as you. So I do think thats odd. But now you know that’s what he expects then just make sure you start the meeting on time. He’s your manager so I guess he can say what he wants to do in a small thing like this.

From your OP

My manager is always in back to back meetings

Quite often meetings can run over slightly, seeing you start your catch up is his cue to wind up the last one or make his excuses and dip out

He was back to back and running late - the OP just wasn’t joining

I just don't really understand why it's an issue. There are so many things to get stressed about at work and this isn't one. In fact, it's an easy win with your boss.

You start it at the start time, not wait for someone else to start it. It shows you're not forward thinking that would do my head in if someone was doing that.

Exactly this

I'd start the call on time. But I think he's being weird if what he's saying is that he won't join until you've started the call.

I wouldn’t expect my team to second guess my diary, join the meeting at the allotted time.

However, I equally have no rules about who starts first. Sometimes it is me, sometimes it is them.

The one that does get me is when my boss starts the meeting 5-10 minutes early, everyone else panics and joins and then it looks like I am late when I am on time.

You haven't been a manager OP... often meetings overlap or you're running between things. You wait until the next meeting is started by someone else to move on from your current activity.
And yes, the more junior, less busy person will be expected to wait on the call until they become available.

Is this your first job working on Teams? It is highly unlikely that your manager expects you to start all meetings. He wants you to be in the meeting from the start time. He is hardly sitting around waiting for the meeting start notification. I would be royally peeved if I joined a meeting with an inductee and they weren't there.