Those are installing, yes. But I think there's no such thing as sideloading vs installing, since, sideloading is a way of installing stuff.

for instance, if I wanted to install an app on my BlackBerry Z30, I needed a special tool running in your browser to do so. So a 2nd device, with a protocol. That second device surely then "just" installed my software. But I would call that sideloading, since I needed a 2nd device that has the way to install stuff on my phone.

if I can just click the apk on my phone to get it installed... then that's installing. But if I need my PC and a special tool (e.g. adb) to do the installation for me (again, 2nd device, custom software etc... and there's no other way) then it's sideloading again.

Installing from Steam or Windows Store is not sideloading, since you don't need anything special. And using a tool that's already on your target device is nothing special IMO.

If Steam would need to run on a seperate device to install games on your PC, that surely can be sideloading. Also, if I buy a Switch game from Nintendo Store webpage and tell it to install it on my Switch, it's sideloading again.

This is how I like to thing of the word sideloading, at least.

But overall, sideloading is installing - so in the end, there's no such thing as sideloading anyway.