There are a ton of lower body exercises that target the quads, but a lot of them involve other muscles as well. A great example of this is the squat as virtually every lower body muscle is involved in completing a full rep.
While I enjoy squatting and the many variations I don't quite feel the quads working like I do with leg extension exercises. For many of us who are working out from home or have minimal equipment at their disposal, you likely don't have a leg extension machine. Personally, I find the leg extension machine to be a little too harsh on my knees and thus I hardly ever use it.
Below are two great bodyweight exercises that directly target the quads and require no additional equipment. Both exercises should be done 21 style with a 7 second hold at the end. If you are unfamiliar with 21 style, you do 7 partial reps from beginning to the middle, 7 partial reps from the end range to the middle, 7 full reps, and a 7 second hold at the end.
As a side note, I did these as a superset for three sets and my quads are still feeling a little tender 3 days later.
Exercise #1 - Heels Elevated Narrow Stance Squats:
Exercise # 2 - Quadruped Knee Extensions: