Clip-ons clip in front of your existing day optic and allow you to maintain your day optic zero under thermal. (As long as your thermal maintains collimation or can be adjusted to do so.)
There are certain things to follow to ensure you are maximizing your efforts to maintain collimation and those are things such as making sure your day scope optical centerpoints are as close as possible in line with the clip-on you are using. i.e., a 1.50" height day optic mount and using the same 1.50" optical height mount on the clip-on to eliminate any potential error that can occur.
Clip-ons are good if you have multiple guns that already have scopes. If you don't, a dedicated thermal weapon sight would likely be a better option. Many dedicated thermal weapon sights have different zeroing profiles you can save, so it can make zeroing for different guns very easy.
Clip-ons come at a cost though. High quality clip-ons are much more expensive than dedicated thermal weapon sights.