If I passed on, will my stocks investment automatically be passed to spouse or children?

Without a will, your estate will be distributed in accordance with the Intestate Succession Act. In your case, your spouse will get 50%, while your children will split the remaining 50% equally. It is not an automatic process and an administrator (typically one of the beneficiaries) will have to distribute the proceeds.

However, you should write a will anyway - it gives you more control over which beneficiaries get what, and you get to appoint an executor instead of having your beneficiaries apply to the court for administration. This can be particularly important if you hold property solely in your own name as your property could be split into multiple parts which is not a very favorable situation!

Some things do not form part of the estate and thus would not require a will to be distributed. These include joint accounts (except tenancy in common), insurance policies with nomination, and CPF balances.

Lastly, since you mentioned stock investment, do note that US estate tax is applicable on excess of 60,000 USD of US situs assets. The tax can range from 18% to 40%, and as such you may want to do the necessary planning so as to avoid such a situation - for example, by shifting from US-domiciled ETFs to Ireland or HK-domiciled ETFs. Do consult a tax advisor if you are unsure.

Better to sign up for a joint account with your spouse/children's name depending on who you want to pass the assets too. This is so that if you pass on, the joint account holder have the rights