You can find the details on the docs:
The basic plan is exactly what basic is in meaning. Very basic.
Environments with a basic plan are scalable, no resources can be added,no horizontal scaling etc.
If you need this kind of functions and features, you require a more advanced plan.
Then the information about the Basic plan are bit “inconsistent” across various places on Mendix.com :((( For example here https://marketplace.mendix.com/link/component/119422, you can find info that teh Basic is scalable!
Also information about file space in Basic vary from page to page: you can find both 1GB and 10GB… what is the right one value?
This are the specs of an environment on a basic plan: