It may seem like a silly question, but it actually isn't (for Unity). If you google "unity download", depending on which site you land on, you need to navigate a bit to land on the proper (free) version...
Anyway, if you're interested in Unity: just go with the free "Personal" plan. It's free until you make more than $100K in a year with your (or your company's) Unity activities.
Download the latest LTS release (it's currently 2021.3.1f1 - next week it may be different, but don't worry about that - don't try to chase the latest versions unless it introduces something you really need). You should install Unity Hub first to manage your Unity versions and projects. Just go here: