We have been in the Linux world ever since, hence it will come at no surprise that we are using Ubuntu as an operating system. The application layer uses different technologies such as PHP (yes we are still doing certain things in PHP but it's being phased out), NodeJS and most importantly GoLang. Database is a MySQL cluster / replication.
We are using micro-services architecture with Docker and Kubernetes and have all kinds of tools such as RabbitMQ message queues and deployment automation via Ansible.
All-in-all our tech stack won't break the bank as we mostly use open source technology that does not expose you to any perpetual license fees such as Oracle-DB or RedHat linux.