A suggestion: use Groups or Components to segregate geometry primitives (edges and faces) into non-interacting “spaces” or contexts. If everything is “loose geometry” at the top-most level of the model hierarchy then edges and faces will merge with each other as you create them. You probably don’t want such “stickiness” in certain situations. For example the fireplace is probably best modeled in its own group, so that you could move it around slightly without having any effect on the adjacent wall.
Creating such groups or components as you go is a very handy practice. For example, before you create the fireplace or other elements, triple-click (with the Select tool, default hot-key is space-bar) on some portion of any wall to select all of the attached geometry edges and faces. Then use Make Group or Make Component. After that, continue to create new geometry for the fireplace, then select all the fireplace bits and pieces and Make Group of that, etc. As you create fireplace bits and pieces you’ll still be able to use SketchUp’s inferencing to snap to guides, edges, etc. but the new geometry won’t merge with existing e=geometry that is enclosed or protected within a group or a component.
When you need to make edits to an existing group or component, just double-click (with the Select tool active) on some portion of it to “open its context” for editing. When done editing a group or component, use the Select tool and just click anywhere outside the faint bounding box of the group or component to close it.
You can nest groups and components, which is also handy. By the way a component is a fancy form of a group, which is useful for managing copies of repeating geometry. When you make a copy of a component, edits to any instance of the component will automatically and instantly replicate to all other instances - pretty cool! Groups lack this feature - when you edit a copy of a group, only that copy is affected.