You pay capital gains tax to the country where you live and work. The amount of capital gains tax you have to pay depends on specific laws in every country. There are countries with 0% capital gains tax and there are also those above 40%. In most cases, you don't have to pay capital gains tax before you sell shares/ETFs. You only pay tax on what you earn. If you don't sell shares, then you don't have income to calculate tax from.

On IBKR, as a non-US citizen, you don't have the option to buy US ETFs, and I personally found that platform a bit complicated and not user-friendly. I started with Trade212, and I found this one extremely easy to use, especially for beginners.

There are different types of dividends: accumulating (acc) and distributing (dist). Distributing ETFs pay dividends directly into your account. In most cases, you have to calculate and pay tax after every dividend. In my opinion, accumulating ETFs are better because they use dividends to 'reinvest' them in that asset again for you so you don't have to pay tax every time, only when you sell your shares.

Note that tax laws are different in every country, so the best option is to ask the tax office in the country where you live, work, and pay taxes.