Apple - How do I ensure my Mac is not sending data to Apple or programmers

You can:

1. Go to System Preferences > Security & Privacy.
2. Select the Privacy tab.
3. Choose Diagnostic & Usage from the list on the left.
4. Make sure the 2 checkboxes on the right are clear.

You can also choose other options from the list on the left (e.g. the Location Services one) to make sure your other privacy settings are okay.

An application-based network filter / firewall such as Little Snitch or Hands Off! is recommended to identify suspicious behavior, though Little Snitch seems trustworthier because they have a legal address; Hands Off! vendor One Periodic on the other hand hide their identity and domain info... guess it's owned by some intel agency...

However, those network monitors might not notify you of connections initiated by services not running under your user name (e.g. those running as root).

There have been efforts to collect phone-home and always-connected behavior by Apple themselves, though I do not have any recent sites at hand other than the apparently obsolete https://fix-macosx.com/ and https://lifehacker.com/lets-talk-about-apples-privacy-issues-1655944758 .

This said, besides that Diagnostic & Usage data mentioned by user242397, there are tons of other services that connect to Apple services.

Right out of my memory:

• Spotlight. If you disable web searches and Spotlight suggestions and lookup in system preferences, you will still have e.g. updates for currency conversions which cannot be disabled as far as I know (but you could block them e.g. via Little Snitch)
• Safari. Much like Spotlight unless you disable all suggestions.
• Siri and dictation. Dictation has an offline mode, but this might still send usage data to Apple.
• apsd, a daemon facilitating push notification services which maintains a connection to Apple to be notified whenever something is directed at you, e.g. a FaceTime call. Therefore Apple will always know your IP address and your identity at any time.
• Handoff / Continuity to sync between your devices, see http://osxdaily.com/2014/10/30/use-handoff-mac-os-x-and-ios/
• iCloud and cloud photo services which also keep a more or less steady identifying connection to provide iCloud services, store notes and even unsaved files located e.g. in or below your Documents folder.
• Mac App Store services (e.g. storeaccountd) which periodically check for updates and malware signatures etc., even if you disabled automatic updates in system preferences
• iTunes which is active when the app is running
• Wi-Fi connectivity & captive portal detection via WISPr. When you connect to a wireless network (even your own ones), your OS will try to load an apple.com page to see if you are online, and if a captive portal page is loaded instead, macOS will present that page for you to sign in.
• Maps and geolocation services.

Oh, and did I mention that Apple collects data about your usage using "differential privacy" beginning with macOS Sierra, which is opt-in according to various news sources (probably via Diagnostics & Usage), but this is anyone's guess and may change over time. In theory, differential privacy ensures that collected data cannot be tracked back to a single user but are cumulated over a group of users, but this only works if there are no flaws in your implementation, and Apple has a track record of being maximally opaque regarding their architectures and designs of such features.