Computer keeps using the wrong default printer

Go to Settings > Bluetooth & devices > Printers & scanners and, under the printer preferences, turn off Let Windows manage my default printer. On Windows 10 it's a checkbox at Settings > Devices > Printers & scanners. When it's on, Windows makes the printer you used most recently the default.

Select the printer you want and choose Set as default. On Windows 10, click the printer, then Manage, then Set as default. The option only appears once automatic management is off.

Select old printers, duplicates or entries with Copy 1 in the name and choose Remove, so they can't be picked by mistake. Keep Microsoft Print to PDF if you use it to save files as PDF.

Some programs, including office suites and web browsers, remember the last printer used in that program. Pick the right printer once in the print window and print; the program should remember it next time.

Open System Settings > Printers & Scanners and set the Default printer menu to your printer instead of Last Printer Used.

Select any printer you no longer have and click Remove Printer.

Press Command + P and confirm the Printer menu shows the right one. Some apps keep their own last choice, so selecting it once usually sticks.