Short answer: When using the program installed on your own computer from a CD or download, each tax return is a separate file, like having separate word processor documents. The files are not shared between computers and are not backed up to the cloud. Only you have copies, no one can see your data unless you share the file.
Long answer: The turbotax license technically allows you to install the program on up to 5 computers that you own, not 5 computers owned by other family members. When you install it the first time, the license key will be linked to your online Turbotax account. The only way someone else can install a copy using the same license key is to have your online password. That will allow them to see any data you saved online. That might include forum posts like this, and your purchase history. It might include old tax returns if you previously used Turbotax online to file your returns, but it won't include 2023 returns for the reason mentioned above. And just because you can work around the installer issue by sharing your password, doesn't make it correct.
