Here's a quick comparison of always-free resources that each cloud vendor offers. It should be obvious this is not a detailed nor an apple-to-apple comparison. The offerings and service names differ between vendors, and so do terms provided by different providers. I only picked the compute, database, and storage options for a quick comparison.
|AWS||750 hours (1yr only)||25 GB DynamoDB||100 GB storage gateway|
|Azure||750 hours (1yr only)||Azure Cosmos DB (25 GB)||5 GB blob storage (1yr only)|
|1 e2-micro instance per month||Firestore (1 GB per project)||5 GB-months of regional storage|
|Oracle||4 ARM-based A1 cores and 24 GB memory||NoSQL DB (3 tables with 25 GB per table)||10 GB object storage|