How to plug old external SSD with Ubuntu installed into new laptop

In order to boot from a USB port, many/most current computers (with UEFI etc.) will require entering UEFI/BIOS settings and enabling the option to boot from the USB port. For a one-time basis, many will allow a "boot menu" (on my Thinkpad, several years old, I can hit F11 during pre-start).

This is a good way to verify if the Linux install on the external SSD can actually boot in this setup, at the least -- but to do it on a routine basis, assuming it works, you'd need to install a bootloader that will allow you to choose between Windows on the internal drive and Ubuntu on the USB port. GRUB can do this, of course, but anything that repairs the Windows 10 install in any way is likely to replace GRUB with the original Windows bootloader (which won't give you the choice, rather just start Windows, at least by default).