We do use a Microsoft Office 365 E3 tenant in my company, we have only 25 licenses but this was offered to use because of a Microsoft partnership as a training center. We have absolutely no connection to our internal infrastructure.
All those licenses are used by the internal staff and our internal trainers.
We now do plan to train external people from other company to the use of Office 365 (OneDrive, SharePoint Online, Teams, Planner,...)
What will be the easiest way to split the everything. Internal Staff on one side, trainee on the other ?
We don't want trainee to be able to see any of our SharePoint site, Office 365 groups,....
I was thinking about a sub domain ( training.company.com ) on a other tenant. Is this possible / best way to do it ? Any idea ?
You'd usually have another tenant for this, that would require additional licences and it would have no connection with your internal tenant. Sub-domains aren't a thing for this function, having a sandbox for training is no bad idea anyway but there are the costs to consider. You can apply for the Office 365 dev program that offers a complementary tenant, also there are Office 365 trials available but whether this workload is considered appropriate for that I can't say.