Hi. I am getting a new M5 Pro MacBook. I currently have a 2015 MacBook Pro. What are some tips on how to sort out data and files on the old one before doing a transfer? Ideally, I don't want to migrate over duplicates, old files, old redundant downloads and apps. Also, any advice on how to best set up the new MacBook before the migration? Please feel free to ask any questions that help you give any tips and answers. Thank you!
Doing a "house cleaning" on your current (i.e., MBP 2015) is a very good idea. Look through and, as desired, trash/delete items you know you no longer need or want. Of course, you can re-organize along the way as well. Documents, Mail, iTunes/Music, Photos, etc.
Install/Download (the current version of) apps if at all feasible. If you need help recalling who the developer/publisher of the software is, which makes it easier to narrow the correct release (i.e., not falling prey to apps with the same name) and legit Web sites/domains, get info on the app and look for the copyright info/attribute.
For most software, you can note the registration key via the About section or similar. And most files related to an app are typically stored independently. Even apps with "libraries" such as Photos, iMovie, and Final Cut Pro are decently easy to find and often just require a double-click to re-import them to the associated app. However, some software has embedded/integrated or otherwise very 'internal' storage systems. As macOS has gained features, capabilities, and whatnot, the sprawl of application support files has widened significantly. Decades ago, it was mostly a PLIST file in the Preferences folder. Mac OS X added areas like a dedicated Application Support folder for associated files beyond presence. Then things spread to containers. Recently, I learned there are also (now) group containers. Basically, a lot of places for files to hide. The app I am referencing and the only one I had any notable difficulty (i.e., exhausting research, trial and error copy-and-pasting) transferring data from was coconutBattery.
Beyond applications, you will need to reconfigure some system level preferences (e.g., Desktop wallpaper, screen saver, display sleep timeout), configure third-party email services, pair BT devices, go through the permission notifications, and app notification permissions dialogs again.
Overall, it is a much more tedious process, but for the most part, should not be more than nuisance tedious. Although, as I mentioned, you may come across some frustrating barriers.
What if they are stored in iCloud?
When you set up your new Mac as 'new'/'fresh' (i.e., skip automatic/assisted migration), you can, and typically should, still sign into your Apple Account/ID during the initial setup process. By doing so, iCloud Drive, Mail (i.e., for your Apple Account email address) are enabled and configured, your Apple Account is automatically signed into apps like App Store, Music, TV, Podcasts, etc. In other words, when you get to the Desktop, even on a freshly set up Mac, iCloud Drive will appear in the Finder (window) sidebar as usual with whatever content has been there.
Interesting. So you save all the documents onto the USB and then as you need them bring them over.
There are several ways. If you keep your old Mac active until you've fully configured it, there will be multiple paths. You can manually move files onto a USB drive, then from the USB drive to the new Mac. You can use an app to generate a cloned version of your previous/old Mac, then copy files from that USB drive. And/or you can use AirDrop, which would be available after signing in (as noted earlier) to the new Mac with the same Apple Account/ID.
I used a combination of the three. And still had the previous/other Mac active, so fairly easy referencing.
P.S.
Once the new MBP has been set up with your new account, connect the cloned backup from the OLD Mac.
Let its icon mount on the desktop. DO NOT "open the icon" yet.
Click on the icon ONE time to select it, then bring up the get info box for it (type command-i).
At the bottom of get info, click the lock icon and enter the password you're using on the NEW MBP.
Put a checkmark into "ignore owership on this volume" (sharing and permissions).
Close get info.
I used SuperDuper, and this attribute was already enabled following a successful clone. I do not know if SD automatically toggles the attribute. I bring it up simply to point out that it may just need verification of being enabled.