How are ETFs, especially actively-managed ETFs, taxed when changing their portfolio?

Every time they sell something that sale is evaluated regarding its tax impact.

For example, let's assume some ETF sell their 1k USD position in stock A, purchase 1k USD of stock B, and sell it two months after.

Selling stock A results in capital gains or losses.

Selling Stock B has capital gains or losses.

The tax impacts are assigned to the investors. In the US these are reflected in the 1099. That is why even if you the investor never sell your shares, your 1099 can still show boxes filled in.

The answer is the same for mutual funds or ETF's.

Now if you have the investment in a IRA, or 401(k) then the capital gains aren't included on a 1099, but they are shown on your quarterly/annual statement.

When investors sell a mutual funds position, fund shares may need to be sold in order to raise cash to meet the redemption. If the latter occurs, that triggers a taxable event for both.

However, if an ETF is making ‘in-kind’ creation and redemption of shares through an authorized participant, the capital gains are not recognized at the time of the transaction and are not considered a taxable sale. If done in a non sheltered account, the investor is taxed when he closes his position.