Does it have the Windows GUI?

Linux Mint’s Cinnamon GUI is very similar. Has a start menu, task bar, tray, etc. It was one of the reasons I chose Mint, as it feels familiar.

Is the file architecture the same (C: Drive, Documents, Program Files, etc.)?

“File System” works the same as “C:” in Windows, it is your root drive. Your personal effects live in the “Home” folder. This is like My Documents in Windows. Everything else is in various places under File System.

Can I migrate my files over through One Drive

Microsoft doesn’t make a Linux GUI for OneDrive. That said, you can get your files through the OneDrive website (simplest) or though a third party GUI people have made for this exact purpose.

Can I use my Word/Excel 2024 there

Microsoft doesn’t make software for Linux (with some exceptions). However, LibreOffice works well on both Windows and Linux (meaning you can try it out right now), and it works well with your spreadsheets and documents.

What’s the alternative to Windows Defender

Unneeded. Viruses are 99% a Windows issue. It’s the only OS that needs active defense like that. There are some options, but I wouldn’t bother with them.

Can I run all my Steam games on it

All of mine do. Check out ProtonDb. Anything rated Gold or higher will just work. Things marked silver will work after some tinkering. And I wouldn’t bother with Bronze.