Spending isn't withdrawing from a balance
In the beginner course you saw that Bitcoin has no account balances, only unspent transaction outputs, or UTXOs, sitting in the ledger waiting to be spent. What that course didn't cover is that when you send Bitcoin, your wallet has to actively pick which UTXOs to use as inputs, and that choice has real consequences. If you have five separate UTXOs of 0.01 BTC each and want to send 0.03 BTC, your wallet might combine three of them, or it might use a single larger one you also hold and send change back to yourself. Different wallets use different selection algorithms, some minimizing the number of inputs, others trying to avoid creating dust, tiny leftover amounts too small to be worth spending later.
This matters because every input you add to a transaction increases its size in bytes, and fees are paid per byte, not per coin. A wallet holding many small UTXOs from years of small payments will pay more in fees to spend them than a wallet holding one large UTXO of the same total value, because it has to reference more inputs and provide more signatures.