I understand that there is 30% withholding taxes on dividends for US-domiciled ETF. But is there any US domiciled ETF that accumulates? That way isn't it all captial gains?

I believe US domiciled ETFs are required to distribute the dividends. Besides, as mentioned already, the withholding tax is applied when companies in the ETF pay dividends to the fund, before it reinvests them.

The WHT is also deducted in an accumulating ETF.

Even though the ETF doesn’t pay out the dividends to you, it still does withhold the tax on your units before reinvestment.

I think this was covered in one of the articles by Seedly. Can do a search on it.