in many countries people have a lot of free time, a lot of creativity, passion etc.. but not a lot of money. this is where the DIY lathe comes in. or alternatively, the DIY pottery wheel. people have built these things from old car parts for many years. I don't think you can do it in your apartment. what you want can be 3d printed for less than $200 from ASA filament. you just need to find someone with a 3d printer that can print ASA in the size that you want. it could take a week to print on high detail. the professional shops charge a lot for that time. some hobby user or makerspace will give it to you for the $50 monthly membership fee + a spool of $40 ASA. fusion 360 is free for student or non-commercial use.

sls nylon will be expensive in that size. you can also look at forming thermoplastics. that is like stamping. a bit different than injection molding. there are techniques that involve spinning a disc while pressing on it to form it. I have only seen it done in metal but I don't see any reason why not to try it with thermoplastics. bowls can be pressed from discs with hot dies. I don't see why you can't do that with a bird bath. the design will be something that works for the technique used. a 3d model (the design) will be different for 3d printing vs injection molding vs stamping. you need to understand what 3d printing can not do. $2000 sls in that size for example. 3d printed parts with overhangs can be a problem. cleaning 3d printed parts can be a problem. moulded PVC is super easy to clean if it has a good surface finish. it is used in vinyl siding. some formulations are good for 100 years outdoors.

the question is why build a bird bath. why not buy it. what are you willing to pay for that bird bath. will you make money back selling your design or your product. if you already own a 3d printer AND you can do 3d design, this is a free bird bath. for everyone else, it is not. that is why for everyone else it is a tool for work. a path to a business that saves time vs traditional methods. that does not guarantee a good product. it only reduces cost for people who make products.