I use MariaDB because its open source and relational database. Out of open source relational database MariaDB stands out providing scalability and availability. Other relational databases needs DBA personas but MariaDB made this easy.
I use gRPC because it is based on http/2 and provides advantages when used at scale. With protobuff grpc will be more efficient and easy to integrate. Grpc can give efficient way to talk to server it can open persistent connections and also asynchronous communication between client and server.
I use Envoy because it supports both http and http/2. It supports grpc and rest. It can even translates REST to Grpc. Best functionality i like from Envoy is it generates metrics which can be imported by Prometheus.
I use GraphQL because it is driven by client and api user defines the input schema which reduces number of end points on backend. End points will be delivered quick. It gives response as per DB schema but this can customised.
I use MongoDB because its document store with json and easy to store and query. Less learning curve. Give lot of query capabilities and next door to relational database.