There are situations with SharePoint Online where businesses wish to restrict users from downloading files. Unfortunately, this can’t be done at a document library level but I can be done at a user level provided you have licenses for Conditional Access.
Conditional Access is a features of Azure AD P1 and is included in SKUs like Microsoft 365 Business Premium. The above video takes you through the steps of configuring an appropriate Conditional Access policy in your environments to prevent downloads. The policy can be targeted at specific users and expanded to include other Microsoft 365 cloud services if desired.