across is described as '1. Bluetooth Keyboard/Mouse Emulator: across makes a Bluetooth equipped PC or Mac work as a standard Bluetooth keyboard/mouse combo' and is an app in the File Sharing category. There are more than 100 alternatives to across for a variety of platforms, including Windows, Mac, Android, Linux and iPhone. The best alternative is AnyDesk. It's not free, so if you're looking for a free alternative, you could try KDE Connect or Chrome Remote Desktop. Other great apps like across are Barrier (Free, Open Source), Pushbullet (Freemium), DWService (Free, Open Source) and UltraVNC (Free, Open Source).
KDE Connect is a project that aims to communicate all your devices. For example, with KDE Connect you can receive your phone notifications on your computer, or just use your phone as a remote control for your desktop.
DWService is an open source project which offers a service that allows access to remote systems (Windows, Mac, Linux, Raspberry...) using a standard web browser - without any component required on the client side.
UltraVNC is easy to use and free software that can display the screen of another computer (via the Internet or a local network) on your own screen. The program allows you to use your mouse and keyboard to control the other computer remotely.
TightVNC is a free remote control software package. With TightVNC, you can see the desktop of a remote machine and control it with your local mouse and keyboard, just like you would do it sitting in the front of that computer.
Synergy combines your desktop devices together in to one cohesive experience. It's software for sharing your mouse and keyboard between multiple computers on your desk. It works on Windows, macOS and Linux.