Even outside of a first-time purchase, a 72-hour withdrawal hold may still apply to some debit/credit card, digital wallet (Apple Pay, Google Pay), or PayPal purchases. Whether a hold applies can vary based on factors like your account history and transaction activity, so holds may not apply to every purchase.
The amount held is equivalent to the purchase amount, not your total account balance. Any additional cash purchases made during an active hold will also be locked for the remainder of that period.
Trading is unaffected — you will still be able to trade throughout the period of the withdrawal hold.