Which broker to use for US ETF e.g. vanguard snp 500? Saxo or td ameritrade. Quite confused by the many costs.. Also, is roboadvisors another “alternative” to invest in us etf? I would like to buy and hold US etf, before dabbling in the rest.?

TD Ameritrade is fine and completely o.k.

no account fees, zero USD commission fees, however company will be merged with Charles Schwab broker in the second half of 2020 if U.S. regulatory authorities do not object. when you send money to brokers do not convert you home currency (f.ex. SGD) to USD vis your bank or home broker but use cheap and safe Transferwise (or currency conversion/sending. You can use that even within Singapore if you want convert/send to an USD account of Singapore bank/broker.

I've been using TD Ameritrade for a number of years and really like the straightforward platform and robust charting tool. No hidden fees as far as I know. Can't comment about saxo as I've not used it in any capacity.

And yes robo advisories are an alternative to invest into Etfs, US domiciled or otherwise. Only thing is you cannot choose which are the etfs you want to buy into. The portfolio will be constructed based on your risk profile that is determined when you open the account and answer their cka questions.