Your question is US tax question so it has been moved to tax now.
"I was told I can send up to 100.000 USD per year to the US from foreign assets with no capital gain and still not declare, is that true?"
The moving of your own money to the US is not a taxable event. The tax law you are referring to is under reporting for gifts made to you from a foreign person. A non US person can give you up to $100,000 in a tax year and you do not have to report the gift (if you are a US person) but if you receive gifts of $100,000 or more you must report using form 3520. This is not a tax calculating form just a reporting form. AND the funds were never your funds previously they are gifts to you.
A US person also is required to report their foreign accounts. That is under FATCA ( Foreign Account Tax Compliance Act ).
FATCA requires certain U.S. taxpayers who hold foreign financial assets with an aggregate value of more than the reporting threshold (at least $50,000) to report information about those assets on Form 8938, which must be attached to the taxpayer’s annual income tax return.
You may have to complete and file other reports about foreign assets, such as FinCEN Form 114, Report of Foreign Bank and Financial Accounts (FBAR) (formerly TD F 90-22.1), in addition to Form 8938.
If you expect to be moving $100,000 each year form your own account abroad the you are surely required to report the account that holds it on form 8938 and FINCEN 114. No tax except on any interest you are earning. The interest is still taxable even if the funds you earned prior to coming to the US are not.
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