When you import the CSV files with latitude and longitude in them - I'm assuming that means values like 51,2 (or other "small" values) - you commit the cardinal sin of lying to QGis about their projection. QGis is very trusting and believes you when you say these values are in metres measured from some distant origin. Thus you get incorrectly placed data (somewhere very close to that origin) and wrong distances.
So when you import the data you have to accept that it is stored in degrees and admit this to QGis. You then need to reproject the data to a projection that is in metres. If you are sure you live in EPSG:32651 then that is a good choice, to reproject your data you select the layer and export the layer to a new file (geopackage is a good choice here) with the new CRS you need (EPSG:32651) - QGis will then add this new file to your project as a new layer. This layer will be in metres and you can proceed accordingly.