Giant Defy Advanced 2 brake replacement I just bought a 2019 Giant Defy Advanced 2, with Giant's "Conduct SL Disc" brake system and it's the only thing about the bike I don't like, for many reasons. But my question is, are the rotor mount holes "universal" i.e. can I replace the Giant system with Shimano 105 mechanical brakes? Any help would be greatly appreciated.
There are two standards to mount disc brake calipers that are relevant to you:


flat mount
post mount


Have a look at Nathan Knudson's explanation to identify what your bike has. Its specs sheet might also help.

These are industry wide standards. A Shimano flat mount will fit, for example, on a bike that came with SRAM flat mount.

There are adapters for flat to post mount available. However, it is better to avoid these. In particular post mount calipers on flat mount frames are quite a bodge.

All rotors and calipers are cross compatible (at least all that are relevant to your case). It will be enough to align the new calipers for existing rotors. However, one needs to check brake pad compatibility. Some (mostly very cheap) rotors must not be used with sintered pads.