The three currencies
Research Points (RP) are your experience: you earn them in battle and spend them to unlock vehicles and the modifications that upgrade them. Silver Lions (SL) are the standard currency for buying researched vehicles, purchasing modules, paying repair bills and training crews. Golden Eagles (GE) are the premium hard currency, bought with real money and used for premium vehicles, premium account time, accelerated research and extra crew slots.
Premium account and premium vehicles
A Premium Account boosts your RP and SL earnings per battle. Premium vehicles earn more RP and SL and can help fund the grind, but they don't research their own tree line. Neither is required to progress — they speed things up.
Modifications and crews
Each vehicle has modifications you unlock with RP and then buy with SL, meaningfully improving performance — a "stock" vehicle is noticeably weaker than a fully-upgraded one. Crew skills are a separate, per-vehicle-slot progression that improves reload, aiming, repair speed and more.
Managing the grind
Repair costs scale with BR, so higher tiers cost more to sustain. The healthiest approach is to progress steadily, keep a profitable low-to-mid-BR lineup you enjoy, and avoid rushing to top tier where a bad game can cost more SL than you earn.