If I have minecraft on my Xbox One as a disc, how can I play minecraft (with mods) on my PC?

Buy the pc version. Isn't that obvious. If you bought the game through Xbox live or the pc digital version then you can play on Xbox or pc. Just need to log in with your Xbox live username and password.

You can either play the Windows 10 version or the Java version.

The Windows 10 version can cross play with Xbox One, while the Java version can't.
Java version can change versions, Windows 10 version can't.
Java edition is often up to date, which is why updates for the Bedrock Edition and PS4 Edition are delayed.

Because you want to play mods on your PC, buy the PC edition, as stated from Bone Alone. It costs roughly $27 USD.

If you have XBox One version, you are NOT ENTITLED to the Windows 10 NOR the Java version & therefore MUST purchase them separately (As of Oct. 19. 2018, the Windows 10 version if no longer bundled with the Java version).

XBox One & Windows 10 versions run on the Bedrock Engine, which don't have any known mod support. Only the Java Edition has mod support, but it's not officially supported, so it's a bit of a DIY deal. The Twitch desktop app does include a Minecraft Launcher that allows you to download & run modded versions of Minecraft: Java Edition with minimal effort.

Please be aware that you must own the Java Edition in order to utilize modded versions.