There are many possible CAD programs that teams can learn, all of which can be downloaded for free under a student or FTC team license.
Here are a couple suggestions to consider:
A fully-featured CAD package, but it runs entirely in the cloud. It can run on any computer (even Chromebooks!) and has iOS, iPadOS, and Android apps as well.
Onshape also has a comprehensive tutorial system (https://learn.onshape.com) that will not only teach you how to use their software, but how to approach design problems.
There is a huge FTC parts library available to teams as well. More info here: https://ftconshape.com/introduction-to-the-ftc-parts-library/
If you’re just starting out with CAD, or you don’t have access to powerful computers, Onshape is the software for you.
A cloud-based all-in-one CAD/CAM package made by Autodesk. Fusion 360 is cross-platform compatible, however, it doesn’t run very well on low-spec computers.
A more noteworthy difference is that Fusion ignores every single industry standard, creating its own structure and organizational system. Beyond simple sketches and extrude features, Fusion’s modeling and assembly system is unique and not compatible with any other CAD software, making it hard to switch away from Fusion.
You can see more options and more details at Game Manual 0.