API or SDK, and pricing

Hi,
I am sorry if this has been answered already. I went through a lot of posts but I couldn’t find a suitable answer to my situation. Hence requesting guidance.

I am building an web marketplace, where many mentors will talk to many students (Strictly 1:1 calls only). How many calls per day/ month is not controlled and can potentially be unlimited.

I wanna track whether both users have joined the meeting on time, when the meeting ended, and collect feedback from both users after the video call.

In this scenario, should I go for an API or SDK?

Is there anything I need to pay at all in either case? Please let me know the price I might have to pay from the platform’s side.

Are there any other terms I must know about before choosing either. I am completely new to this and would be properly guided by some handholding here.

I remember reading somewhere in this forum that the REST API is completely free for 1:1 calls. Is that so?