Android Open Source Project (AOSP) is FOSS, but it is not copyleft - almost everything in AOSP (aside from the Linux kernel, which is copyleft under GPLv2) is licensed under "permissive" FOSS licenses which allow anyone to take it, change it, and distribute non-free derivatives.

The Android that comes on a phone you buy in the store is (with very few exceptions) largely not FOSS - it is a bunch of closed source proprietary bits running on top of something derived from AOSP. On some phones it is relatively easy to replace the Android it came with with a version that is mostly FOSS, but on many phones it is not.

In theory, the fact that Android distributors are required to make their changes to the kernel itself available (because it is copyleft) should make it easier for people to make FOSS operating systems (Android or otherwise) for these phones, but, for a variety of reasons, in practice it often doesn't work out that way.