Is it Coinbase.com which is custodial wallet or Coinbase wallet which is noncustododial wallet that generates you seed phrase, keys and your own addresses?
Coinbase.comIt is custodial wallet, not you keys and not your coins on blockchain. Coinbase has the full control over your own coin and they can decide to freeze your Coinbase account as they pleases.
Coinbase walletThe wallet will generate you seed phrase (12 words seed phrase), keys and addresses.
Not also recommendabke as it supports cloud seed phrase backup, although you can decide not to backup your seed phrase on online cloud but many people can do this which is highly not secure and safe at all.
Backing up on online cloud is not safe. Backup offline is recommendable but replicate it and have it backup in (like 2 or 3) different locations.
To hold large sum of coins, use cold wallet like paper wallet,
Electrum cold storage, or for convenience and altcoin support, use
open source hardware wallet.