On chain Bitcoin is spread across the blockchain in addresses and and confirmed on blocks as unspent transaction outputs (utxo). Your wallet displays an aggregate total of all these utxo that are spread across the blockchain and in various addresses. They are all separate chunks of sats.
So let's say you want to send .01 Bitcoin but you have utxo denominated in .001 utxo sets. To send the .01 your wallet will take ten sets of .001, and send them all to one address. The more utxo, the more data required which can raise your fee.
If you had .1 BTC in a single utxo and wanted to send .01, the entire .1 is sent, .09 is returned to your wallet on a new address and .01 is sent to the recipient....
I may have made that more confusing and poorly worded than needed 😂