here’s an exercise you can do for yourself. take any meditation subject that can produce first jhāna, and try to map it into the 7sb (satta bojjhanga).
for example, doing brahma-vihara, 16 APS (anapansati), 31asubha (31 body parts), are 3 common meditation topics.
so here’s an example.
i start with metta meditation. this is vitakka of first jhana, equivalent to samma sati and sati-sambojjhanga selecting metta as the topic.
then i start radiating metta in different spatial directions. this is vicara of first jhana, and the dhamma-vicaya-sambojjhanga in action.
i’m starting to get real high and happy, mentally and physically, i decide to switch to 16 APS. So vitakka of first jhana did the task of switching channels from metta to 16 APS. Now vicara of first jhana can evaluate qualities of the breath as it feels in my anatomical body. qualities such as piti, sukha, comfort, leg discomfort, heavinesss, percpetions of earth-element, etc.
if i know how to deeply relax and enjoy body and mind, i can ride this into jhana, or samadhi-sambojjhanga quickly in sequence of the 7sb. If i havne’t figured how to deeply relax, fulfilling passadhi-sambojjhanga, then i’m stuck doing satipatthana and dhamma-vicaya-sambojjhanga without getting into jhana.
if i’m already very experienced at getting into first jhana, 2nd jhana, the buddha talks about doing an undirected type of samadhi development (somewhere in SN 46 bojjhanga samyutta), as opposed to the directed type, which is what SN 46.3, SN 46.2, the cook sutta in SN 47, etc.
so in the undirected samadhi development, you can basically jump right into samadhi-bojjhanga, which has attention to samatha-nimittam (sign of stillness) and abyagga-nimittam (sign of non-distraction) as the main fuel source.
what vism. and ajahn brahm’s explanation for vitakka and vicara of first jhana is more applicable to this undirected samadhi development for one already skilled in samadhi. The Buddha, on the other hand, is trying to lay out a detailed gradual training in samadhi.
First jhana in this regard is particularly important, because it’s training you how to use your mind in the right way, in a skillful way, to see hindrances, the causes, and with skill and practice learn how to overcome them. With a sufficient amount of physical and mental passadhi, that skill can take you all the way into the fourth jhana.