Shouldn't be "ovat" (järvi and sauna)?
Not in the case of [there is (something) somewhere] - siellä/täällä/tuolla/jossain on (jotakin). If the statement was e.g. "The lake and (the) sauna are over here." then it would be "Järvi ja sauna ovat täällä."
What's the difference between Täällä and Tuolla?
Same as the difference between "here" and "there".
Why "on" instead of "ovat", considering that there are multiple subjects? Or do you always use a singular verb in these "existential clauses"?
Above in the comment written by Hacu. it explains why, and in my opinion it is the best explanation.
What is the material difference between
Järvi ja sauna ovat täällä
Täällä on järvi ja sauna
There is a lake and a sauna over here.