You usually use constructions lines when: it's super detailed, or you are drawing from imagination and you want a certain composition and/or focal point or you are first learning to draw anatomy, is when you should mainly use construction lines.
For anatomy whilst learning construction lines, it would be better to understand the measurements of natural anatomy, rather than constructing the image as such.
>do I have to use the Loomis method or just lines to see how the features are placed compared to the others?
Both, if you are still learning, but I would say the later would help you more. Usually I only use a cross sometimes or very VERY simple line gestures for construction if I do need it, like the cross image/line on a face.
But, if you are drawing freehanded from a reference for a personal study, you should not really be using any construction lines and instead focus on your own natural visual observational measurement skills.
I don't know if you are drawing from IRL or just from a tablet with computer images of models, but drawing from life with pen, pencil and paper will help a lot. The skills of drawing from IRL transfer much easier over, than vice versa with a tablet. That, and you will not always be in places to practice with your computer and tablet, where a simple sketchbook and pen and pencil would be better suited.
Drawing from life: drawing whilst on a hiking or camping trip, urban areas, coffee shop (you can do both here I suppose), draw a park, a forest, shrubbery, flowers, leaves, nature, animals at the zoo, drawing gestures of people on a bus, etc.