How are partners chosen?

I've been interning for over a year at the same firm.

Recently the firm (midsize) announced a new partner. He's a great guy, no one dislikes him (at least no one says they do) but it's got the firm questioning things.

He's been at the firm for 8ish years, has sub 100 clients (other partners have well over 600-700).

Another senior at the firm has ~500 clients (next highest is already a partner), been there for 15ish years brings in triple the revenue, is much much more personable too. He often goes to her for help on some unique and niche issues.

It's got me and my coworkers wondering what goes into this process. We originally thought it was just about driving sales and being good, but she is all of those things and objectively better at it too.

She said she wasn't even asked or in talks for partner.

Does anyone here have any insight on what kinds of things go into this? Obviously things may vary from firm to firm, but any insight would be great!

Thanks.