Snap is not needed for this kind of tools.
Watch the command line title (Command:) and the mouse icon on the bottom status bar when tools are active.
There you see hints what is the expected next action, resp. the current left/right mouse button functions.
This helps to understand the workflow of tools and in which state it currently is.
For Fillet, after selecting the first entity, just move the mouse near the second entity and you will see a preview appear.
Try this with some 90° lines and a valid, big radius, because sometimes the result is not obviously visible, depending on parameters and zoom level.
When you see the preview, the fillet is created when you left click at the current position.
For Trim there is no preview, but there is no need to hit the entities exactly, there should be a generous tolerance.
For many tools it also depends, where you click the 2nd or following entities.
The results may differ, if you click the one or the other end or side of a line or a different quadrant of an arc or circle.
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