@stargazer English clubs aren't giving out contracts as such. The top English players are on central contracts with the RFU, but England regularly pick uncontracted players as well. The rest of the players in the Premier 15s are amateur.
Players come to England because they want to play in the best coached / highest standard women's league, and either combine it with study or work in the UK or take a sabbatical to do so - they don't have issues getting released for international squads because they aren't being paid to play by the clubs.
Canadian International lock Emma Taylor works in projects for the Canadian Navy. She took a 6 month unpaid sabbatical from work to join Premier 15s champions Saracens in 2020 to prepare herself for the expected 2021 World Cup. After playing in the 2021 Premier 15s final, she went back to her day job in Canada. Basically anyone good enough will be welcomed to play as I understand it.
It will be interesting to watch the quality of Super Rugby Aupiki compared to the top of the table clashes and knock out stage of the Premier 15s.