Lots of undiagnosed autistic people in the comments describing autistic masking and burnout.

Autistic burnout is the name for the unexplainable energy drain when you have to interact with people, because while social interactions are usually as effortless and as easy as lifting a finger for neurotypical people, they can feel like lifting a 100lbs stone for you, because you have to constantly remember how to act and at the same time focus on the moment to not miss a social cue, etc. This is called masking and it drains your energy.

Now watch people under this comment claiming they are not autistic, because how they are able to perfectly socialize, because they have learned to keep eye contact by putting a reminder on a post-it note on their desk for a year in the middle school.

Before discovering I'm autistic I was totally dumbfounded: why I was tired as if I ran a marathon after a meetup with friends for just a few hours? Now I know.

What you are describing in paragraph 2 is not specific to autistic people, although it is common. Introverts in particular can also become drained from social activity. There is some overlap.

Masking also is not necessarily the source (or sole source) of social exhaustion and instead the exhaustion may come due to greater sensory sensitivity and/or cognitive processing requirements, which are common in autism. Furthermore; masking has other purposes such as avoiding bullying, social exclusion, and other forms of negative outcomes; the intent and effort varies by the thing being masked and the person.

YMMV; it's a spectrum.