As others have stated matrix and XMPP are good options. I myself am an admitted matrix fan boy, so i lean heavy that way....but the choice is yours. For matrix, i know that beyond the mobile and web browser clients, there are several regular, (non-browser), good ol' fashion desktop style applications (scroll down on this url: https://matrix.org/clients/) for linux, windows and Mac operating systems. The story is likely similar on XMPP side, but i have not used xmpp in many years.

Signal works on desktop nicely, but you still need the mobile app on a smart phone in order for it to work.

On computer is generally a _much_ easier case to design for since it can be assumed that the computer is available most of the time to receive messages. Mobile is significantly harder because most of the time it's not available to receive. This ends up in a situation where caching servers are a required necessity, but have to be built to be private. From that, computers become a derivative case for an increasingly niche minority of users/use cases

My family uses Matrix. The default Element interface works in the browser, on desktop, and on mobile app. It was chosen as it's pretty much the only solution that is open-source, supports all devices, has E2EE, supports text/voice/video, and can be selfhosted.

Tox has several desktop clients available, even including a CLI one.

https://tox.chat

https://github.com/Jfreegman/toxic

Session works pretty well on desktop, as well. It doesn't tie to your phone like signal does.