On Sunday, I was walking in Zürich and came across St. Jakob Church. When I approached the door, I saw that a chorus was performing. Since there was nobody around except the people seated in the church, I hesitated to enter.
If that happened at Fraumünster or Grossmünster, probably there would be lots of tourists around and I would follow the crowd. I didn't have that chance at St. Jakob.
Maybe the same rules don't apply to all churches, but I wonder what the best practice is.
It can be a rehearsal, a concert, a regular religious service or even a wedding or baptism.
As @phoog mentions in his answer, if there is nobody at the door to ask your money you can enter.
When there is a service going on, it will be appreciated if you keep out of the area where the people for the service are, in a busy service please stay near the door, if it is a small group in a side chapel or near an altar, you can move around in most of the church.
In the case of a concert, you are asked to stay out of the area unless you sit down and start being one of the group.
Rehearsals are less formal, just do not mingle into the choir or go between the choir and the people which are with them (likely in the seats near.)
Look for notices, they should be obvious but can be to the side or on the other door from the one you used.
And keep your noise down, even more than you would in an empty church.
There are some differences between churches, more between denominations than between different countries. And there are also differences between city and countryside. You can expect a church in a city to be more open to tourists, in the countryside there are still churches which are open every day out of tradition.
Some denominations are more likely to have churches open than others, like Roman Catholic churches have a good tradition of being open for people to come in to pray, while most of the protestant Christian churches are only open for services and to admit tourists. But this is not a given, as each church will decide its own opening times.
If it's a Christian church on a Sunday, it's likely to be a service. Interrupting those, at least as a sightseer rather than a member of the congregation, is likely to be unwelcome. If it's not a Sunday, it may well be a rehearsal for a concert.
As a semi-professional musician, I do concerts like this most weekends, many of them in churches. I personally like it very much when visitors come in during rehearsals. Religious buildings get an extra dimension when they are filled with liturgical music, and the more people that can enjoy it, the better. I can't remember when I last did a rehearsal in a church without a few sightseers.
If you do decide to go in, it is more important than anything else to be quiet. Make sure your phone is set to silent, and don't answer it if it rings. Don't talk to the people you're with in anything above a whisper. For my money, you don't have to sit there; walking around is fine, but not if your footsteps are audible. The orchestra and choir may well have met that day for the first time, and they will have only a couple of hours to put the programme together. The conductor will be trying to get his or her requirements across to about two hundred musicians in a resonant acoustic, and anything that makes that harder is unwelcome.
If there's nobody at the door regulating entrance to the church, then you can assume that the performance is either a concert with no admission fee or a religious service. In either case it would generally be acceptable for you to enter the church.
You tagged "switzerland" but did not mention the denomination of the church.
If it is a Christian church, it should be free to enter. Historically their doors were always open, but in the late 20th century theft became rife, and churches began to lock their doors.
Apart from that, anybody should be able to enter any Christian church, no matter what their own religion, or none, provided they are respectful.
There are tourists who go around blindly, photographing everything but seeing and understanding nothing. I hope your way is more sympathetic and experiential.