Your carrier app should tell you. Also you can pick and choose which apps use cellular.

Settings > Cellular > Cellular Data Options > Low Data Mode

Nope

Low data mode helps reduce data usage, and some carriers support the feature that tells the iPhone your data plan, so you are aware when you exceed it etc, but most do not and you have to rely on the carriers own app to track the usage

Unfortunately No .

The data tracker under settings>cellular>current period is pretty accurate. Reset it at the end of every billing cycle and look at it once in awhile.