Are advertising bonuses from financial apps tax-free?

Apps in the financial sector, in particular, offer commissions for referring friends and acquaintances. Examples include Bison (€15), Crypto.com ($25), or Scalable Capital (€25). Are the commissions I can receive always tax-free for me or are there certain regulations such as a maximum value that is below a tax-free limit?

Is this legal?

It's about manufactured spending. I have the Crypto.com Obsidian debit card. Is it legal if I send my friend €50,000 via PayPal and thus receive the 8% cashback, or $4,000? He sends it back to me, and I liquidate it and transfer it back to my Crypto.com account. Or I use it to buy more…