First, I'll say that you should take the time to get to know the features of the OS, before running out and getting a bunch of 3rd party utilities.
In terms of built in stuff you may or may not already know about if coming from another platform...
- Spotlight: (⌘space) launch apps, universal search, basic calculator, etc
- QuickLook: (space when a file is selected in Finder, space again to close it) Preview a wide variety of files. This sometimes extends into apps as well, like previewing attachments in Outlook or Mail.
- Preview: This app is much more powerful than it lets on.
- Markup: Do basic annotation of images or documents in Preview or QuickLook.
- TextEdit has a plain text mode, I've seen countless "authorities" in Mac education get this wrong. The option to toggle rich text vs plain text is in the Format menu. The settings let you choose the default type of file when you open a new file. It's not going to be your code editor or anything, but it's good to know.
- Image Capture can pull photos off a camera or iPhone, if you don't want to use Photos, or some random 3rd party app.
- Script Editor: For a long time this only supported Apple Script, which made me think it was going to die. But they quietly updated it to support Javascript, so you can do some local desktop automation with Javascript now. Thought I don't think it's as well supported as it once was in some of Apple's new apps. Of course Shortcuts is also there for a more visual scripting option.
- Finder: Show the Path Bar and Status Bar (in the View menu). This used to be the default for the OS, and I think it still should be.
- Not sure if there is a name for it, but you can copy text out of images and videos. This has been a game changer for me. At work, when someone is sharing a website, I can screenshot the presentation, QuickLook the image, and click on the URL to open it in my browser. I do this almost daily.
In terms of 3rd party apps...
- NotePlan: I use this instead of more traditional To Do apps. I can plan based on the year, month, week, or day, with a free form text doc I can arrange how I want, with templates I can design. The files are plain text docs, so I'm not locked in. Obsidian can do something similar with enough plugins, but this seems cleaner to me.
- Plex/PlexAmp: I have a Plex server running on a NAS. PlexAmp can use used on my mac or phone to play music in my library.
- Hush: A Safari extension to turn off all those cookie prompts on websites
- IINA: Video player for when Quicktime can't handle something. I switch away from VLC, because I got so tired of it adding everything to a playlist, and there is no way to turn it off. With IINA I can simple open a file, watch it, and close it. A video player doesn't need to be any more than that.
- 1Password: Password manager. I've been using it basically forever. They seem to keep up on things pretty well and I haven't been left wanting.
- Firefox: An alternative to Safari (my primary browser) when I need it. I don't want any Chromium browsers on my system.
I perged a lot of 3rd party stuff a while back and these are some of the things that made the cut.