When you sign-up with Yamgo and create your Hedera account your Private key and mnemonic are securely generated on your device. Yamgo never has access to your Hedera account.
Yamgo does not store any private keys or mnemonic phrases. In fact, it is entirely possible to use the application without ever sharing your private key. This also means, however, that it is your responsibility as a user to properly store your private key and mnemonic phrase, as losing these means any funds or tokens associated with the account will be lost forever.
It is very important that you save and secure all of the information related to your Hedera Account. If you lose your Hedera Account information then you will permanently lose access to all of your rewards.