How were troops recruited in the ancient times?

The Shastras recognise six forms of Military Troops, a system that appears to have persisted till the 9th Century CE over most of Bharatavarsha. I'm using Kamandaki's system, though not using the original Sanskrit to avoid reporting by Liberals, Lemurs, and Ambedkarites.

1- State troops.

These were Standing Armies recruited by Provincial government officials and paid in coin directly by State employees, though in later years, it appears that land grants became common. However most of these Troops typically came from the Agrarian hinterlands of great cities and were often following in their fathers' footsteps. Most were either Cavalry or Infantry, with the King or Emperor keeping Elephants and Engineers directly under his own oversight.

These were the most loyal and professional of all forces available to a Bharata Lord however with high costs to raise and maintain. Equipping a standard Gupta Heavy Cavalryman would've taken the equivalent of INR 45+ Lakhs today.

2- Mercenaries.

Hated by all clear-thinking Scholars, Mercenaries were however prized for their ready availability, lack of need for external oversight, and often great military experience. It's likely that most Mercenary bands were roving bands of primarily Cavalrymen, Professional Troops from other States themselves, seeking foreign employment for their own reasons.

Kamandaki recommend that Mercenaries be utilised for risky manoeuvres, and kept an eye on at all times. Any Lord with significant wealth could hire them if he put the word out.

3- Guild Troops.

Classical Bharata cities were dominated by massive Guild Troops, with powers extending over multiple State lines. While distrusted by States for multiple reasons, they represented a standing military power of vital note during times of conflict. More importantly, Guild troops represented a significant chunk of urban military might since the scholars knew that degenerate, soft urbanites made for bad troops.

Any Lord desirous of hiring them could get in touch with his City Head who'd then presumably contact the Guild Head. Seals of such persons have been discovered in great number; presumably the larger trade centres held entire cohorts of such troops hired piecemeal by merchants across the continent. The Guild Troops Captain was to be accorded the Status of a Cohort general in the Army.

The Shastras recommend that Guild Troops be used for resettlement of ruined areas, colonization of wilderness, extending Trade networks into borders. Their marginally greater loyalty to the State compared to Mercenaries however was offset by their lesser effectiveness, and thus encouraging strife between the two was as “pleasant as watching a hog and a dog fight.”

4- Militia/ Prisoners of War/ Bribed troops.

Never to be used unless in periods of great crisis, these troops are either difficult to manage or untrustworthy to the extreme or are the mark of a desperate State.

A strong espionage force is of vital importance here, both to keep a keen eye on Provincial and District administration and their military preparedness as well as to subvert the enemy's troops.

Keep them around Professional troops at all times lest they break ranks and flee or lose cohesion and run amok or, worse, join the enemy at the high pitch of the Battle.

5- State and Feudal Auxiliaries.

A wise King, cognizant of the decline of the Age, would ally or vassalize his neighbours but must be ever vigilant about them. A king, while rushed to when respected, simply lost respect by the very act of receiving aid.

Thus the Auxiliary, be it drawn from Allies or Vassals, had to be closely observed and only taken in during times of strength when the need was to shield one's Professionals from harm or when the State possessed little coin to pay mercenaries with.

6- Tribal Auxiliaries.

Fierce and undisciplined, tribal auxiliaries could be as effective as State Professionals at points; however their indiscipline and covetous nature made them a double-edged blade even in the best of times.

The Shastras recommend that they be used for scouting and clearing the way for the Van upto a distance of sixteen kilometres, but preferably not for formal manoeuvres. Regular gifts and good relations with tribal princes and lords- including provision of education, training, hosting, and good Brahmin scholars- would ensure their aid during times of conflict.