ONAN story time again – quant equity. This is completely irrelevant to the gigantic macro issues of the day, so feel free to waste your time in a journey into multiplying matrices.
Arguably one of the first people to do this was Ed Thorpe – in his book at the very end he mentions starting a systematic investing effort though the fund got wound down before it really took off. amazon.com/dp/B07ZWJFYW5
I'm going to skip CAPM and modern portfolio theory because you probably should already know them
The paper that created the well-known size and value factors is ‘Common risk factors in the returns on stocks and bonds’ by Fama and French in 1992. rady.ucsd.edu/faculty/direct…
In retrospect it’s incredibly simplistic: they broke down the US equity market into two groups for size and three for value: (S) small and (B) big for mktcap/size and (L) lowest 30% (M) middle 40% (H) high for price/book. 
They created six portfolios: SL, SM, SH, BL, BM, BH. Their long-short factors become (SL+SM+SH – BL+BL+BH) for size and (SH+BH – SL+BL) for value.
They regressed these long-short factors against stock returns and *handwave* found they have explanatory power. 20 years later they get a Nobel.
I’m not going to go into details, but the main factors that have been accepted in academia and practice other than size and value have been momentum, volatility, and to some extent, profitability or quality.
The main momentum paper is Jegadeesh and Titman 1993 ‘Returns to Buying Winners and Selling Losers: Implications for Stock Market Efficiency’, Cliff Asness claims he invented momentum but who knows who was first business.unr.edu/faculty/liuc/f…
The volatility paper is Ang, Hodrick, Xing, Zhang 2006, ‘The Cross‐Section of Volatility and Expected Returns’ nber.org/papers/w10852.…
Quality or profitability is hard to define so here is one by Fama/French. I’m lazy and don't know the metric, something like ROIC; I guess its that high retained earnings that are reinvested in high ROIC businesses grow quickly? Am I a compounder bro now? papers.ssrn.com/sol3/papers.cf…
