Alternative Salesforce-oriented CI/CD servers
Jenkins is very flexible and is used to build and deploy applications and services written in a large variety of languages and platforms. I mainly chose this server as it is one I had personal experience with; however, you can see here that Salesforce includes details on configuring other CI/CD servers: https://developer.salesforce.com/docs/atlas.en-us.sfdx_dev.meta/sfdx_dev/sfdx_dev_ci.htm.
Jenkins and other similar CI/CD services offer a lot of flexibility with the ability to use scripts, industry tools, and programming languages. However, they do require a place to run them or paying a provider to run them for you.
There are, however, several managed CI/CD server products that specialize in Salesforce, and some even run in your Salesforce org. These are typically also less complex to set up and maintain as they follow the general design goal of the platform itself, to abstract as much complexity away from the administrator...