It has been a while since I’ve taken my probability and statistics class so take this with a grain of salt.
You are correct, T tests are used for a small n while z tests are used on a large n. BUT, t tests require you to use the sample variance while z tests require you to use the population variance. So they require different numbers. But I also remember our professor telling us that if we dont have a a table and just a t table to use the t table with infinite degrees of freedom to get our z value. So they are kind of interchangeable. But there is a difference and follow that rule to get more accurate results. But also 50 isnt the definined line between z and t tests. Some people consider 40 a big n and will use that as their boundary when to switch between z and t.
Just remember it’s been a while since I’ve learned this so you might want to fact check this. I only posted because I didnt see anyone else and wanted to help a little.