In Switzerland, your employer decides which pension fund to use for your compulsory pillar 2a occupational pension plan. Most employers are members of a single pension fund and subscribe all of their employees to that fund.
In some cases, several different pension plans may be available from the same pension fund. If a certain type of plan (such as a defined benefit plan) would work better for you, it is worth mentioning that to your employer. However, which plan you get is your employers decision to make.
Besides tax-preferred pillar 2a savings, you may choose to join an additional private pension fund. In Switzerland, non-compulsory pension funds generally work within the pillar 2b or pillar 3b frameworks. You have complete freedom to apply for membership in any non-obligatory pension fund which suits you.