People who do not apply for asylum are in higher risk of being detained by the police and fined for illegal residency.
Serbian legislation proclaims illegal entry and illegal residency as misdemeanor acts, which can be fined with up to 150.000 RSD (1.250 EUR) or up to 60 days in jail. This means that if you are caught by the police, they can bring you to one of the Misdemeanor courts and the judge will give you a financial fine. If you are unable to pay this fine, it will be replaced with jail sentence. You can be sent to one of many jails in Serbia to serve this sentence. The list of jails in Serbia and contacts can be found on this link.
Have in mind that you are not entitled to free legal aid in misdemeanor procedure, so if you want to appeal you will have to find and pay a lawyer yourself. The deadline for the appeal is very short (8 days) so if you wish to appeal, you have to act immediately. Also, you will have a right to one phone call from jail, but your mobile phone will be temporarily taken away from you and you will not be allowed to use it while in jail.
However, according to the law, asylum seekers are excused from the responsability of illegal border crossing. If you are caught in illegal border crossing, you have the right to seek asylum (at any point in Serbia and during every interaction with the police or the authorities, so even in court), this would exempt you from the fine/prison. Nevertheless, people are often in practice denied the right to seek asylum. In this case, you have the right to contact a lawyer.
Besides the financial fine or jail sentence in the misdemeanor procedure, you can also be placed in one of the detention centers based on the order of the police. Detention centers (Prihvatiliste za strance) only accommodated foreigners and there are no Serbian detainees in these facilities. Serbia has 3 detention centers (in Padinska skela, in Plandiste and in Dimitrovgrad) and they are all managed by the Ministry of Interior. You can be detained in detention center for the maximum period of 180 days. Same as with misdemeanor procedure, you are not entitled to free legal aid so you have to find and pay a lawyer by yourself if you wish to appeal. People who are placed in detention center have limited access to their mobile phones (usually few times per week).
You can be detained by the police in the detention center for different reasons, such as not having identification documents, you didn’t leave Serbia after you got expulsion order, you gave fake identification information, you entered Serbia illegally (without visa and passport), etc. The legal purpose of detention center is to detain people prior to deporting them back to North Macedonia/Bulgaria or back to the country of origin. However, most of the people who are detained are not deported and they are released after the period of 180 days expires. But this depends on the location where the person was caught by the police, nationality of the person, security risks, and many other circumstances, so it is not possible to predict in advance if you will be deported or released within those 180 days.