How do conferences detect double submission?
"Actually care about it": Read the conference's rules.
"Can detect": Irrelevant. Like many things in academic research, this is primarily honor system. If your teammate thinks that whether something is acceptable is determined by whether you can get away with it, they need a serious attitude adjustment.
By the way: do you have a faculty advisor on this project? They need to know about this. They would also be the best one to give you advice on the specific norms of conferences and publishing in your fields.
Organizers of conferences with overlapping review periods sometimes exchange the submission information to detect cases of multiple submission. However, since doing so without the authors' agreement would be a breach of confidentiality, this can only be done ethically by conferences who announce this process in their call for papers (In my field, that is the case for several top conferences, including FSE 2019 and ICSE 2020).
There are several other ways how the duplicate submission would eventually be discovered. Let's say the paper has been submitted in parallel at two conferences called A and B.
There could be a shared reviewer for conferences A and B who notices the duplicate submission. In this case, the reviewer would probably contact the chairs of both conferences, leading to an immediate disqualification of the paper. Note that such a reviewer would not have to be a program committee member at both conferences: program committee members often delegate their workload to colleagues in order to reduce their workload.
If the paper is accepted at conference A, the paper is eventually published in A's proceedings. In this case, the reviewers from conference B might become aware of the submission of the paper to A. Consequently, they could contact the chairs of conference A to enforce its retraction.