I havent studied the new proposal (apparently its for 2028 but still needs to be approved into law). Under the current system, you declare your assets in your yearly tax filing. You can of course try to leave certain assets out, but that would be illegal and the tax filing program is pre-filled more and more each year (so they have data links to banks etc).
If you go for the standard route, you are taxed based on the assumed return per asset class that you have invested in. So if you have 1K of savings, you declare that amount and that 1K in savings is assumed to have returned 1.44%, so you get taxed on 14,40 of assumed savings income.
Similarly, if you have 10K of equity or other assets (BTC falls in that category), you declare that amount and that is assumed to have earned 5.88%, so you get taxed on 588 of assumed income.
They then add up the assumed income from all asset categories and subtract the assumed interest on certain types of debt. That is the 'assumed net return' on which you are taxed 36% (this is all based on the 2025 rules). Oh and I cant forget to mention, the first 58K (single) or 115K (couple) of net assets (you can again subtract certain types of debt) is untaxed.
There is also a second option next to this 'assumed return approach'. If your actual return is lower than those assumed returns, you can opt out of the standard approach and have them tax you based on your actual returns.
Havent tried that myself yet though, so I dont know the full details there. There's not that much to tax on my end, so havent felt the need to dive into it 🥸 And I even think they are planning to automatically use this approach if your actual returns are lower than the assumed ones from 2025 or 2026 onwards.