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Calling metadata api using just session id

I have an apex class which class external service. This serivce is supposed to load some metadata when called.
I was thinking that I will pass user's session id to external service, and using just session id it will be able to call that metadata api. But after searching on net for hours, I didn't find any post which allows you to call metadata api just based on session id.

Isn't it possible?

If not possible, then what are the alternates I have?

How does apps like conga composer access salesforce data without asking for username and password? Do they do OAuth?