At age 20, how and what should I invest in with extra cash of about 10k?

A. Personally, I feel you can increase your robo-advisor monthly contribution as 1) you are starting from a very low base of $50/month & 2) you had a monthly salary to draw upon before university starts. If your investment amount in robo-advisor is too low, might not justified the management fees incurred & unable to reap too much upside when equity market recovered a few years down the road.

B. Since you are still young, you can afford to to take some moderate risks (do your own due diligence!) in investing local stocks just to get your feet wet. Rmb blue chips companies can easily soured into "bruise" chips if their biz/revenue model faced long-term headwinds. Only invest money you are comfortable in losing/stomach unrealised paper losses & never use leverage.

C. Since you alr started working, I assume you had a CPF account alr set up. Whenever you had spare cash in excess of expenses & investment purposes, do consider topping up your CPF SA and/or transfering OA balance to SA to take advantage of the early compounding effects of high interest (up to 5% in SA). This would give you a headstart among your peers by the time you graduate by virtue of having a financial safety net set up. This will allow you to be more aggressive in investing when you had a full-time job & more income aka ammunition

The easiest way to get started would be unit trust or ETF. They provide diversification and industry or regional focus strategy. This investment allows you to save time which could be put better use such as pursuing your hobbies/passion or even side income. Just make sure you read up on Marco-economics.