Steve is right, generally speaking. But if your frame does not allow for internal cable and component runs, the Di2 upgrade path is a bit more complicated. Also, for Di2, the seat post needs to house the battery in older frames. Not all seat posts can. The alternative is an external battery placed under the down-tube near the bottom bracket....again using discontinued Di2 parts. With a SRAM upgrade, if your present bike is 11 speed and you want to keep your crankset, you’ll need to purchase SRAM Red from 2019 or 2018. SRAM has moved to a 12 speed standard for 2020, which requires a different crankset. You’ll also be replacing brakes and hoods/shifters with SRAM.
Moving from 105 to Di2, you’ll be replacing hoods (shifters), front and rear derailleurs and installing several electronic components. You’ll probably end up replacing brakes as well because they typically come with the Di2 upgrade package. It sounds harder to do than it really is. But, it can get expensive.